That'd still be a GB game, just one that requires SNES + SGB. Not a SNES game that requires SGB + GB flashcart. Ask yourself "what is 90% of the game written in? What cpu does it run on?"
Viewing post in Making a SNES game that works only with a SNES accessory?
In the gameplay idea I have in mind, the code will mostly run on the SGB CPU indeed, but the input, and audio and graphical output will be SNES related. I know it's kind of a stretch, but I see it quite similar to using a SA-1 chip or SuperFX chip, which the compo rules allows. And the fact to use an accessory like the SGB is tied to the "perspectives" theme :).