[API]

Today I was looking through my server browser, and noticed the lack of tags for game mode indication. I would assume this primarily comes from the fact that sv_tags is a cvar, and I think people are scared of messing with it. With the code here, you will be able to ensure that a tag is always included for your game mode or plugin. Please note this isn't the only way to do this, and probably isn't the most efficient either, but it works.
I didn't code in support for multiple tags per plugin (or at least I didn't intend to)... this can be accomplished easily, but I didn't design this specific example with it in mind. To be honest, one tag is all a plugin should really allocate in order to show it's presence.

#define CUSTOM_TAG "mycoolplugin"
new bool:KeepMyMouthShut;
new Handle:hTagsCvar;

public OnPluginStart()
{
    hTagsCvar = FindConVar("sv_tags");
    if(hTagsCvar)
    {
        HookConVarChange(hTagsCvar, CallbackTags);
        KeepMyMouthShut = false;
    }
    else
    {
        SetFailState("Error attempting to hook sv_tags! This game doesn't support it?");
    }
}

EnsureTagsInclusion(const String:IncludeMe[], const String:CurrentValue[])
{
    new String:CurVal[512];
    strcopy(CurVal,512,CurrentValue);
    // Always just fix their tiny formatting errors.
    ReplaceString(CurVal, 512, " ,",",",false);
    ReplaceString(CurVal, 512, ", ",",",false);
    // Trim it too.
    TrimString(CurVal);
    // Are we empty? If we are, then automatically append.
    // Otherwise, check it and append only if necessary.
    if(strlen(CurVal)==0)
    {
        SetConVarString(hTagsCvar, IncludeMe);
    }
    else
    {
        // Now lets get all the current values.
        // An easy way to do this is to explode by the comma.
        new String:TheStrings[64][64];
        new ExpCount = ExplodeString(CurVal, ",", TheStrings, 64, 64);
        new bool:IsFound = false;
        for(new i=0;i<ExpCount;i++)
        {
            if(StrEqual(TheStrings[i], IncludeMe, false))
            {
                IsFound = true;
                break;
            }
        }
        if(!IsFound)
        {
            Format(CurVal, 512, "%s, %s", CurVal, IncludeMe);
            SetConVarString(hTagsCvar, CurVal);
        }
    }
}

public CallbackTags(Handle:convar, const String:oldValue[], const String:newValue[])
{
    if(!KeepMyMouthShut)
    {
        KeepMyMouthShut = true;
        EnsureTagsInclusion(CUSTOM_TAG, newValue);
        KeepMyMouthShut = false;
    }
}

public OnMapStart()
{
    new String:CurrentValue[512];
    GetConVarString(hTagsCvar, CurrentValue, 512);
    EnsureTagsInclusion(CUSTOM_TAG, CurrentValue);
}

Oh... by the way... I haven't tested it. Maybe someone else can?