So I was looking at ways of making objects appear when a lot is unloaded, and the LargeObjects flag (S3PE --> OBJD --> Grid --> ObjectTypeFlags --> [12] LargeObject --> true) does exactly that.

Unfortunately, it has the secondary effect of having the object appear on all floors, so this conversation seemed potentially fruitful:
https://forums.thesims.com/en_uS/di...oof-decorations
http://simswiki.info/wiki.php?title...4F1D#Misc_Flags

As such, the apparent solution seems to be setting these flags:

S3PE --> OBJD --> Grid --> ObjectTypeFlags --> [12] LargeObject --> True
S3PE --> OBJD --> Grid --> ObjectTypeFlags2 --> [11] IsGreenhouse --> True

Tried it out on EA's greenhouse roof, and it seemed to work, but once I reloaded it, the same problems appeared as before- the greenhouse object appeared on all floors!

Any thoughts and corroboration? For the record, I have included the edited EA greenhouse piece as a reference.