Found this over at halomods fourm. Looks very promising if you ask me
Xbox bsp editing

Whoa this does look promising.

I think there are 2 BSP's in any given level, the model bsp, and the collision model bsp.

hot damn! if that hill is walkable that would be awesome!

Looking through the points (and not really understanding them) I decided to replace everything between the offsets with random hex. The result was the coll model being all screwed up. So just a stab in the dark but i think that between those offsets is the coll model. If we could move those points in relation to the blood gulch model points, then i think a custom map maybe possible.

I say post your ideas, because I (like many) would like to see a custom map

Bump