I can't really give you anything definate here, the way I did it with the forts was to copy the stone fort data into the basic vanilla fort (in a modfolder) and overwrote it. Then played around with the text in .worldpkdgdescr. I lost count of the number of CTD's I got before it worked and there were several bad results, a half buried fort and floating fort.
So my guess on this is that custom battle uses the default settlement;
Entry from my modded wooden to stone fort.
Code:
serialization::archive knightly_order_fort_b
settlement ` Settlements/South_European/ambient_settlements/fort/se_fort_a.world culture southern_european environment generic faction any fortificationlevel 3 type default_fort variant stone_fort_b
It should be possible to change the 'variant' to point to the settlement you want.
Its a lot of trial and error, but I'm very interested in this. Please keep me posted and know I'm happy to help out if needed.