I just realized, I can set all kinds of weird RENDER_FORMATs using GetSetProjectInfo_String and they will return true, though they make no sense.
Like the "Dude"-format, I invented in this example, returning "RHVkZQ==", unencoded "Dude" as set render-format.
Code:
A,B=reaper.GetSetProjectInfo_String(0, "RENDER_FORMAT", "Dude", true)
I think, this function should return false if I try to set an invalid format.
The Render to File-dialog reverts to WAV as standard, so there's no problem over there.