It's all been done before. Check out United Noobs Vehicle mod for COD2:
http://forum.i3d.net/modifications-c...icles-mod.html
Developed in 2006, and released to the public in 2007. Tanks, jeeps, planes, and Flak 88. 2 man jeeps - one drives, the other man's an MG.
FYI - the guy who made the mod is the same guy who did the first xmodel exporter. His website is DieagoLogic.com. I am pleased to say he was a personal friend of mine, and we did many mods together. One of them was custom multiplayer maps for COD2 long before the mod tools came out:
http://callofduty.filefront.com/file...p_Pack_1;56488
To accomplish them, El Diablo made a small program which stripped out all the SP entities from a SP map. Effectively it left it free to run on a MP server, once the MP spawns and level script had been added.