This is now solved. I found a small portable DVD player that supports USB and HDSC input. It's a generic brand. The folders and files are listed on the screen, so navigating and selecting can be done from some distance away. I hadn't looked at portable devices previously because I assumed that they would not have a remote control - I don't know how many of them do, because it seems superfluous in a portable. But the combination of a 7" screen and a remote, together with the memory device source, fits my requirements exactly.
This unit also has a coaxial digital connection to the amp, so I am not concerned that this is a cheap device - the quality seems fine.