One more data point - snagged a cheap G fare BOS-MSP-ICN and called in to check for MUA availability, agent knew straightaway what I was trying to do but unfortunately there was no OY space available (despite flight being Z4) so was waitlisted instead. (Waitlist also appears to have been applied correctly based on how the booking now looks.) Entire process took ~5 minutes and was surprisingly painless, but I guess I got lucky with an experienced agent