I have 0.00% knowledge of MainStage. But if I understand what you want happening, your sounds stop then the reverb return just tails out?
Unaware if someone has suggested something similar, I just tried this out and here’s my gig file. Works great. I used Valhalla Supermassive with a seconds long 100% wet tail.
If you play in Lostin70s ep patch in rackspace1 - play real loud - then switch to rackspace 2 the reverb tail just continues. Set your tail time in Valhalla Supermassive to taste. You can wait or there’s a LostIn70s clavi patch set up there that you can just play over the existing reverb tail.
Gets slightly trickier (but only slightly) but you could also have a manual override MIDI controller knob for that reverb tail if you needed it.
tail-thru-global.gig (30.3 KB)