Been messing around with rockboy on my Clip+, seems to run respectably although Faceball 2000 crashes and reboots whenever you hit Start or Select which makes it impossible to get in-game (EDIT2: verified same behaviour on desktop versions, not RB's problem).
That aside, I was wondering if there's any chance of getting an unscaled viewing mode; obviously I'm aware of the screen's size limitations, but here's an example of a game which (if it worked) would be functional without scaling:
By having an unscaled view and allowing user panning (perhaps toggling volume up/down for screen up/down in the menu?) a fair few games could be made to be playable where otherwise what's going on on-screen is unidentifiable.
EDIT3: I tried to get an unscaled view going on smaller-than-original displays, but wasn't able to get any meaningful results. I managed as much as getting the scaling menu to appear in Rockboy before stalling completely; I focussed my efforts on menu.c and lcd.c, but ultimately changing the settings did nothing to the way the game was displayed when running. This was my first time even trying to compile anything Rockbox-related, so it's probably not all that difficult to accomplish for anyone who knew what they were doing; just wasn't for me yet.
EDIT1: Incidentally, there's a tool called GBS2GB (which I won't link to, as the homepage also features ripped Game Boy music and that's legally questionable at best) which converts GBS files--the ripped format for Game Boy/Color music--into .GB ROMs for playback in emulators or on hardware; while the small screen of the Clip and similar series make a lot of games unplayable, having GB support adds the side effect of rudimentary GBS playback; the sound quality isn't perfect, but since the GBS codec hasn't been worked on in years and isn't included in general use builds, this is one alternative way to go about it.