Netty, with this lavatory meaning, is in a few dictionaries: Oxford online, Collins, Wiktionary. It can also mean netted, or net-like; or neat, natty. I've also seen it used by an Australian as a diminutive for netball.
An example, cited by the OED, from the
Independent in 1992:
Our toilet was an outside netty shared between two or three families, where you sat on a hole and hoped the cat wouldn't jump at your backside.
But I find
netty is already accepted in Chi, always has been. Presumably for the adjective meaning net-like, as the plural
netties is not allowed. I'll add this plural, but not the alternate spelling of the singular,
nettie, as this seems to be seldom used.