What you are asking for is easy to do using Stripper.
https://forums.alliedmods.net/showthread.php?t=39439
If you know what Stripper is and have been using it then all I have to say is attached is an example stripper file for No Mercy that shows you how easy it is to do.
How it works:
To start the finale in No Mercy you press the radio.
For this map the trigger on the radio starts up disabled so it can’t be pressed.
Something in the map enables it once all the survivors are in the finale area.
All I did was use Stripper to start up the radio with the trigger enabled so it can be pressed any time even by one survivor as the only survivor in the finale area.
Note the actual name of the final map for No Mercy is:
c8m5_rooftop.bsp
Here is the Stripper cfg file for that map that enables the radio.
c8m5_rooftop.cfg
PHP Code:
modify:
{
match:
{
"hammerid" "3086885"
}
replace:
{
"StartDisabled" "0"
}
}
If you don’t know what stripper is then follow steps 1,2 and 3 in this post to get it installed and most importantly step #3 to check that you installed Metamod and Stripper correctly and Stripper is it running,
https://forums.alliedmods.net/showthread.php?t=334216
Then all you have to do is put the attached file in:
\L4D2\left4dead2\addons\stripper\maps\c8m5_ro oftop.cfg
If you want to attempt doing one yourself here are the steps.
Start a game and go to the finale or map you want to change.
Go to your server’s console and type
Stripper_dump
or if you have rcon access set up type this from the client console
Rcon stripper_dump
This will create a file that has all of the entities for the map.
When I did this for No Mercy finale I got this file in the stripper dumps folder
\L4D2\left4dead2\addons\stripper\ dumps \c8m5_rooftop.cfg
You then open that file in a text editor and look for
"classname" "trigger_finale"
That is the entity to change.
If you get Stripper installed and my example working and just want a few more maps done post a request with the map names in the stripper forum and I will generate the cfg files for you.
https://forums.alliedmods.net/showthread.php?t=39439
But really give it a try. It is worth the effort and fun to learn how to do mods on your own.