I recently found a $20 DS lite for sale and I am wondering if flashcarts would be allowed for runs on the leaderboard since it is using the game's rom but running it on original hardware

probably not bcz who knows if the loads are going to differ but who's gonna be able to tell if you're using one?

I know that using nds bootstrap on a 3ds has faster loads, so it may be the same with a flash cart. No one has tested the loads on one though