OK, so it’s not insane assuming you don’t have any knowledge at all of what you’re asking for and have assumed that it’s an easy and automated process. I’m not aware of any automated tools for this functionality, so yeah, it does mean loading up the game and taking one or more screenshots. Many games switch between multiple borders as you play, which means you’d need to get at least that far in the game to capture the border, e.g. Pokemon Trading Card Game actually shows your current badges (or something; I’ve never played it) in the border, so there’s borders for every possible badge combination.
There’s a total of somewhere over 650 games with Super Game Boy support, counting all region duplicates. Obviously, that’s less if you strip out games released in US/Europe/Japan/etc., but that’s not always going to be correct as a number of games have different borders based on region–e.g. probably every Pokemon game, weird conversions like Bomberman GB (Japan) -> Wario Blast (USA, Europe), etc. Then there’s games that are Super Game Boy compatible but don’t actually provide borders, which you probably won’t know anyway until you test them, so they take almost as much effort for no results.
If you do decide to organize for a group of people to start releasing borders, then that’s admirable, and clearly a lot more reasonable than asking SuperrSonic to do all 650+. Most emulators with Super Game Boy support should be fine for capturing screenshots. BGB, bsnes/higan and VBA-M should all do the job, among others. (Make sure to set your colors to their uncorrected versions in emulators that attempt to duplicate the original color output).
I think this would be cool too, although I’m not sure if the RetroArch authors would want to take on the potential legal burden of distributing tens or even hundreds of copyrighted border artworks. But who knows; it’s unlikely anyone’s going to create legal trouble over some old Super Game Boy borders, so they might consider it a non-issue. Still, given the trouble they had over on Android, they may be wary. Even if it’s not possible to distribute these with RetroArch, a single package here on the boards would certainly be sensible.