Let's put it this way: I'm a programmer, have a decent understanding of the NES, even better understanding of mappers themselves and know 6502 ASM. Soon after after I got my PowerPak, I was wondering the same thing. So I took a look at the sources and quickly got frightened and closed them, never to look again!
I suppose this is mostly because I don't have a clue about file format used for this, but I know others who do and even they have trouble with it. Props to bunnyboy for knowing how to do this shit! I can certainly see why we don't have MMC5 support yet