Well, you can make a flashcard yourself, but it would be huge and heavy, hard work, and in the end, more expensive than the "original".
As for the firmware, yes, you can rewrite the firmware of anything with an upgradeable firmware; take the GBAMP CF for example. One of the first flashcards to allow homebrew, thanks to Chishm completely rewriting the firmware. You can do this yourself with the R4 or M3, but it requires l33t h4x0rZin' 5ki11Z.