Yes, I mean that stat would return some error. EIO or similar.
Ugh. OK. Welp that's out then.
Seems like the out of band reconfiguration of mergerfs might be the best then. Unless just guessing that a "slow down" error is a quota error and allowing people to set some timeout. It'd be super clunky but it might work. But then it's not much different from the out of band method.