So for FO3, I could enable the ESM flag in an esp file using FO3Edit, create another esp file dependent on the esmified one, then open the dependent esp in the GECK and go to work making an add-on esp.

I just tried doing the same thing with FNVEdit and the FNV GECK, but it's not working: the GECK stalls while loading, saying 'Not Responding'. I did set bAllowMultipleMasterLoads to 1 in GECKCustom.ini, so it's not that...

Has anyone else been able to do this successfully? I could use just FNVEdit to do what I need here, but certain things are of course easier to do in the GECK (just as others are easier in FNVEdit).

Ugh - nevermind, you can disregard this. The problem turned out to be that when I created the dependent esp in FNVEdit, the esmified esp file was added as a master, but FalloutNV.esm itself wasn't. Pretty sure it didn't work that way in FO3Edit... (but I could be wrong, and it may have changed in more recent versions of the program anyway).

In any case, I added FalloutNV.esm as a master, and now it all works properly.