I'm not sure what plSpawnMod is used for, or how it differs from plSpawnModifier, but it doesn't seem to have getters or setters for any of its properties. It can be read from a PRP or PRC, and written out to either, but not modified at all.
Is this a matter of "not implemented until PrpShop supports modifying directly" (rather than in PRC), or is there another reason they're not implemented.
I don't need them for anything, I just noticed they were unimplemented.