I think it would be fun to meet the occasional over-powered monster in a slayer area. A fiend-blood troll in the Cerulean Hills, a beholder in searing heights, a dragon in the fens. They would be limited to the single instance, so the party could either call in reinforcements or flee, giving up the extra treasure chest. Or (gasp!) use a little PnP intelligence to tackle it.
From a coding viewpoint, this shouldn't be difficult. Just add some very-low probability entries in some tables?