Well, some resources of onslaught maps are still left in BFBC2, but not everything.
There are probably major parts missing, not only in the files but also in the code of the game. I haven't looked into this very much, but I think it's very hard if not impossible to get onslaught running like on the Consoles.
There might be a way to use the onslaught pathfinding files in conjunction with existing maps to at least get a "feeling" of Onslaught.
Personally I wouldn't count too much on it.
There used to be a signature here.