OP here.

Just to follow up, I was able to exchange $340 in quarters no problem at the change kiosk at the Bellagio.

I had also asked at the main cage at Flamingo, and was told that they would take them if I was a registered guest at the hotel. (It seemed odd that they would tie it to staying at the hotel, but that's what she said. I didn't press her on it, since I didn't have the quarters on me at the time, and I had earlier seen the counting machine in the Bellagio kiosk).