Thanks for the reply Baker,
I'll try Quoth if I can't get it to work through TrenchBroom.
I found this TB addon, however:
http://leray.proboards.com/thread/3653/add-custom-models-trenchbroom-1
There's not much of an explanation, but the script format (in the def file) looks like this:
@PointClass base(Models) model(":progs/models@bits2@banner@tris.mdl") size(-16 -16 0, 16 16 52) = models@bits2@banner@tris : "" []
So, I thought I could try simply substituting the file path to that of my own.
Loading the def file in TB allows me to place an entity named after my file (just an empty cube), but loading up the map (in Quakespasm) I can't find any model.
(Also, what's up with the @'s in the file paths? Some python convention?)