Never thought of securing the ROS and doing it the other way. I will give it a go but I suspect that the randomness of the movement will make it quite hard to hold. Maybe I can make some kind of jig to hold it against.
My main concern with the belt and disc machines was mentioned - my understanding is they are not really designed for finishing and are uni-directional in their sanding (although oscillating might help). Although if people are using them effectively with worn sheets that is enouraging.
I was even looking at the trade tools belt sander which is designed to convert to a mini desk sander:
RI1200BS Renegade Industrial B/Sander 100 X 610MM Heavy Duty Belt | Hand-Held Power Tools - TradeTools | Huge Range, Great Service
The advantage of this is variable speed control which is not generally found on the smaller dedicated belt/disc sanders.