I've got no clue how Vel and BlipBloop managed to knock out a custom vehicle, abilities and all, before custom vehicle support came out, but they did it! Check it out right here-
Mods are generally SUPER specific to version, but since it's just Assembly-CSharp and resources files (super easy readme in the DL link :) ) it's worth a shot. If it doesn't work, ya can always just redownload Beta 5 :P