So I downloaded FF Hackster and took a crack at making a Final Fantasy I Boss Rush Mode. I downloaded some .sav file that gives you an impossibly powerful setup, and I ran through it, and everything seems to work. Here's a YouTube link:
Basically, I modified the tiles in the Temple of Fiends to enter boss battles in the sequence you normally encounter them through the game. Not all of the bosses are included, but most are, and you can't fight Chaos at the end because finishing the battle with him ends the game. When you do make it to the end of the boss rush, you get a second Masamune.
Any feedback? Suggestions? With some sophisticated hacking techniques that I don't really know about, I'm sure there's some potential to make this pretty cool.