That's right, you can now use up to 18 players by changing the new l4d_maxplayers cvar. It works both on Windows and on Linux, and it will report the correct # of players in HLSW.
The only plugin I know of right now that seems to use these extra slots is my own l4dready (it unlocks the spectate command). Please let me know what other plugins use the extra slots (by default it will only allow 4 survivors, 4 infected per team) so I can add them to the list on the original post.
(Also added a left4downtown_version cvar that plugins could check).
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A huge thanks goes out to ivailosp who figured out the Windows addresses to unlock slots and spurred me into developing this fully featured functionality.
Thanks also goes out to DDRKhat for letting me use his Linux server to test this mod (we got up to 15 players before people started getting bored and leaving ;)).
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For the next version I will try to concentrate on adding more developer functionality such as some of the functions requested so far.