Hi, I see things in a similar way, and think that a "SGB only" game should be allowed by the current rule as it's now clearly forbidden.
My intent is to make a SGB only game, that mainly run on the SGB and not a SNES Rom embedded in the SGB. I want to try to use the SGB feature regarding display (colors, the fact that you can display part of the border over the gameplay area, etc.) and audio (the library of embedded sound and audio engine) in a unique way, in a way that no commercial game did it during the 90's. A GB cart that can't be played on a GB would make no commercial sense then, but for such a retro development I think it could be a cool project :).
But I wanted to ask if it was OK before going too far in the project, and I can see that not everyone is ok with this "edge cas". I hope we can have an official answer from the compo organizer to see if it's ok or not.
Regarding accessibility, I actually though of it too, and I think requiring a SGB is less an issue than requiring a SNES mouse or a Super Scope. For one, the SGB is perfectly emulated in the latest versions of BSNES and Higan. And for real hardware, if you don't have SGB + GB flashcart, you can actually play the game directly on the SNES with a SD2SNES / FXPAK Flashcart, as it have a SGB core now :).