It has crossed my mind.... after all, there isn't much else to do with regard to tankmates. Having said that, some years ago I've had Endlers raid angelfish eggs despite the parents standing guard. I think I'd have to remove the Endlers to give gulper spawn a chance. But they're the only thing that keep the tank lively LOL
Asterophysus have not, but other auchenipterids of a somewhat similar nature have been spawned. They merely interlock pectoral/dorsal fins, and the male partially inserts his seminating organ into the female for internal fertilisation. Eggs should be large and covered in a thick gelatinous mucous of sorts; I doubt endlers would eat them quickly at all.