After a few unsuccessful attempts to just use whatever Magos editor tells me is in the Naga building models, I decided to look in different mpq archives. Turns out, the war3x.mpq Magos uses is different from the war3x.mpq of the game, and the model attachment path was different, which is why the model refused to work.
Apparently the real Naga birth model is in "buildings\Naga\CoralBed\NagaBirth_Small.MDL" (different one for every Naga building) and not in the "SharedModels" folder...