Thanks guys. It would actually be a fairly simple feature to add, but I'm afraid misuse and confusion surrounding it would be rampant. "Why didn't LaunchBox move all my games for me?" "Why didn't it automatically find them when I moved them over here?" "Why did LaunchBox not find my ROM when I renamed the file?" "Do I move the files first, and then change them in LaunchBox, or the other way around?" "Why didn't my images move with my games?" "Where did the game manual files go?"
It's certainly feasible, but a good, clear solution needs thought about and designed before jumping in. Relative paths can also be a challenge, because we have emulators, ROMs, videos, music files, manuals, and images all using relative paths. So the solution unfortunately isn't as simple as "so yeah, I moved my game files over here instead."