If you want to try that, I would recommend one of those self-inflating camping pads. When you roll them up and squeeze the air out they are a reasonable size, while a regular mattress topper would be pretty big to carry.Or could passengers bring their own toppers (probably sleeping bag-size, to fit the typical roomette bunk) as a DIY interim fix, the way some passengers bring their own pillows?
I know I can't sleep well without a decent memory foam, so that's what I would try.