[Kailo]

Update: 2.0 released. Check changelog below.
How? Dark Magic!

Used a few tricks i created functions for variable arguments access.

This workable only with non-array variables for now (write if it needed to add arrays/strings support).

Instruction:
1) Include valist.inc in plugin with #include directive (You must include it first among all includes for proper work).
2) Call va_start(int param_n) in variable argument function, where param_n number of argument with ..., counting of arguments start with 1.
Note for 1st version: It's important call va_start before any variable will declared. Overwise, it'll work not correct.
(In 1st version need to call va_start first in function, in 2nd not).
3) For getting next argument use va_arg(). In case if no more arguments to get, it'll throw error.
You can use va_args() for getting count of params passed in function and va_more() to check if all arguments getted.
4) When you no more needed to get arguments, you must call va_end(). Overwise, memory leaks.

Offtop: Please, if you see any grammar mistakes, notice me about it. I'm not going well in English.

Plugin example:

PHP Code:

// © Maxim "Kailo" Telezhenko, 2017

#pragma newdecls required
#pragma semicolon 1

#include <valist>

public void OnPluginStart()
{
    RegConsoleCmd("sm_nums", Cmd_Nums);
}

public Action Cmd_Nums(int client, int args)
{
    ReplyToCommand(client, "Sum: %i", Sum(10, 40, 50, -30, 80));

    return Plugin_Handled;
}

int Sum(int ...)
{
    va_start(1);
    PrintToServer("args: %i", va_args());
    int sum = 0;
    while (va_more())
        sum += va_arg();
    va_end();
    return sum;
} 


valist.inc code

PHP Code:

// © Maxim "Kailo" Telezhenko, 2017
// Version 2

// Struct of valist
// int arg_count;    // 0
// int location;    // 1
// int iterator;    // 2
// total:            // 3
enum
{
    m_arg_count = 0,
    m_location = 1,
    m_iterator = 2,
};

// Global anchor for memory access
stock static int g_anchor[1] = 1337; // =)

// Allow to get variable local address in plugin memory
stock static int GetAddr(any[] array)
{
    int anchor[1];
    return GetAddrWorker(anchor, array);
}

// "any ..." needed for escape warning "symbol is never used" in previous function
stock static int GetAddrWorker(int[] anchor, any ...)
{
    return anchor[4];
}

stock static void GetOldFrameBase(int[] anchor)
{
    anchor[0] = anchor[2];
}

// Allow to get value form plugin memory (DAT, heap, stack) with local address
stock static int Deref(int local_addr)
{
    return DerefWorker(g_anchor, local_addr);
}

// Will throw error if anchor_addr bigger than local_addr
// Therefore: valist must be included first for lower anchor_addr
stock static int DerefWorker(int[] anchor, int local_addr)
{
    static int anchor_addr = -1;
    if (anchor_addr == -1)
    {
        anchor_addr = GetAddr(g_anchor);
    }
    return anchor[(local_addr - anchor_addr) / 4];
}

#define THIS (view_as<ArrayList>(this))
#define member_set(%0,%1,%2) %0.Set(0, %2, %1, false)
#define member_get(%0,%1) %0.Get(0, %1, false)
methodmap Valist
{
    property int arg_count
    {
        public get() { return member_get(THIS,m_arg_count); }
    }

    public Valist(int param_n)
    {
        int anchor[1];
        GetOldFrameBase(anchor); // result stored in anchor[0]
        ArrayList base = new ArrayList(3, 1);
        member_set(base,m_arg_count,Deref(anchor[0] + 8));
        member_set(base,m_location,anchor[0] + 12);
        member_set(base,m_iterator,param_n);
        return view_as<Valist>(base);
    }

    public void Close()
    {
        THIS.Close();
    }

    public any Arg()
    {
        int iterator = member_get(THIS,m_iterator);
        if (iterator > member_get(THIS,m_arg_count))
            ThrowError("End of parameters reached.");
        member_set(THIS,m_iterator,iterator + 1);
        return Deref(Deref(member_get(THIS,m_location) + (iterator - 1) * 4));
    }

    public bool More()
    {
        return member_get(THIS,m_iterator) <= member_get(THIS,m_arg_count);
    }
}
#undef THIS

#define va_start(%0) Valist VALIST = Valist(%0)

// if have more args to read
#define va_more() VALIST.More()

#define va_arg() VALIST.Arg()

#define va_args() VALIST.arg_count

// Must called when list no more needed
#define va_end() VALIST.Close() 


Attention!
1st version not working in 1.9 since sourcepawn "Disallow negative indices in all array access patterns" (d7743c3) update.

Changelog:
v2: Changelog
v1: First release.

// Download latest version below in attachments.
Old versions:
v1: valist.inc

How i did this: (tricks)
1) Unchecked memory access
Then you pass array as param, plugin know only address and check index for negative value

PHP Code:

{
    int array[1];
    int var1 = 50;
    PrintToServer("%i %i", Fn(array), var1);
}

int Fn(int[] array)
{
    return array[1]; // will return var1 value
} 


2) Knowledge of SourcePawn virtual machine
Plugin memory сonsists of DAT (globals, statics, consts), heap, stack (order as i wrote).
When functions calls:
1) Pass arguments in reverse order
2) Pass num of arguments
3) pass old frame address
4) pass 0
Using sipmle trick i can dump stack

PHP Code:

void Fn()
{
    int a[1] = {777};
    dumpstack(a);
}

stock void dumpstack(int[] anchor)
{
    for (int a = 0; a <= 15; a++)
        PrintToServer("%i: %i", a, anchor[a]);
} 


Using this in process of writing include i got ():

etc.
-8: 2220   // unused memory
-7: 20780 // unused memory
-6: 20776 // unused memory
-5: -5       // value of a
-4: 0        // just 0
-3: 20804 // old frame // for example look at {1}
-2: 1        // param count
-1: 20800 // 1st param; address of a // Here is started vm_start() call
0: 666     // a[1] = {666} // address of this array is 20800
/// {1} Here beging frame of vm_start(), here address is 20804
1: 0        // just 0
2: 20824 // old frame
3: 1        // param count
4: 1        // 1st param // Here is started vm_start() call
5: 0        // Valist VALIST variable
6: 0        // just 0
7: 20856 // old frame
8: 5        // arg count in Sum call
9: 2264   // 1st param
10: 2260 // 2nd param
11: 2256 // 3rd param
12: 2252 // 4th param
13: 2248 // 5th param // Here is started Sum() call; All 5 params is heap address
14: 2236 // this heap addr of where will stored result of Sum() for call ReplyToCommand in future
15: 20876  // this  addr of "Sum: %i" for call ReplyToCommand in future

All addresses is local for plugin memory.
Also as we see stack reversed in memory.
memory: DAT-start ... DAT-end, heap-start ... heap-end, stack-end ... stack-start;
3) pseudo-class based on methodmap + ArrayList
4) Some macros for good looking code