I've liked this game for while, and am thinking of causally running it on my free time, but I wanted to know if a SuperCard SD with the GBA loader on Gamecube (to stream and record while playing on a CRT) is a valid way of running the game?

Assume not allowed if it's not gbp/gbi/legal emulator. If you want to argue for it, join the discord, and be ready to prepare tests compared to existing cart runs for accuracy.

Since you said you just want to rum it casually, I wouldn't worry much about leaderboard submission when starting out - but you asking about it is appreciated!​