Y’might look at geeqie. It has a collection feature that might scratch your itch…
@carvac, My thinking about how a collection would work would be a list of file paths to renditions scattered around my central photo directory hierarchy. The software could take such a list and display it like the contents of any directory, with all the available operations, 'cept maybe ‘move’. That’s how I’d keep track of my portfolio, for instance…