Kudos to the fellows at Halowar, who passed along an interesting tidbit regarding a Halo mod for the HalfLife engine (strange as that may seem). The details, for your reading delight:

My name is Jason and me and my dev team are currently making a HALO mod for Half-life. With the news that Counterstrike has vehicles in there mod, this has opened the door for us, now we can import the one vehicle that ive been wanting to make for ever and thats the buggy from HALO... of course there will be more vehicles. This is somewhat Halo news, because we are doing Halo but in a different engine and perspective... or course its not going to be exactly like Halo but it has its theme... plus we are looking for more members to join this mission. If you do decide to post this as news about Halo, please put that we are looking for members to join the team. We are looking for anyone with Halflife SDK experience... also coding experience. Modelers and mappers *MUST* have a Halo intrest. Contact is ganlas@hotmail.com or icq# 85999038.

A tall order, to be sure--much of the charm of Halo's vehicles is in the engine, which may be beyond the ability of a modded HalfLife engine. Nevertheless, that hasn't stopped people in the past... if this sounds like your kind of project, don't hesitate to get on board.