Even if, like for BF2, you can patch (cough hack intellectual property) the game to change what server it has to ping to enable "online" play, it will still needs a, as you say, third party benevolent group and we still won't be able to do what I was trying to do, which is a basic, fully offline LAN gameplay.
My friends and I often play BF2 remotely on a vLAN through a VPN, so we are quite used to BF2 networks tricks, and I thought we could trick the game, no luck.
If you can get your hands on the very first, unpatched version, it should work. I got that to run in an offline LAN setting.
EDIT: I remember now what the problem was, running a co-op local game. They'd allow you to do a LAN game, but not with bots. A friend of mine played it locally in single-player, and I'd use my online account to connect with IP, a button which was only unlocked if you used an online account. You may be able to work around it by specifying the IP address to connect to in the windows shortcut for launching the game.
EDIT2: I may or may have not been using a crack at the time, not sure if that has anything to do with requiring an online connection...
AFAIK, vanilla BF2 has more or less been abandoned by the BF2 community for this.
I'm not sure how they reworked the entire thing, so I'm guessing you might find the source-code/reverse-engineered stuff somewhere.
"Project Reality: Battlefield 2 is a free-to-download single and multiplayer game modification for the Battlefield 2 (BF2) retail Microsoft Windows PC game. Project Reality installs alongside your existing Battlefield 2 game installation, allowing both the original game and Project Reality to be played separately without conflict."
My friends and I often play BF2 remotely on a vLAN through a VPN, so we are quite used to BF2 networks tricks, and I thought we could trick the game, no luck.