Bringing this back from the dead - but big thanks to you for posting this. I have used a lot of this info and expanded upon it to work on modifying madden 08/12 for PSP. It's been extremely useful to get me started. I posted some additional tutorials expanding upon these for Madden 08 editing over on Operation Sports (since it's more console-centric).
I was able to expand roster sizes significantly to over 2150 players from the original 1962 players in Madden 08, though wasn't successful in doing this in Madden 12. It seems Madden 12 is a bit more tricky to do significant editing than 08 is, from everything I've tried. Madden 08 seems to follow along more with how Madden PC editing works and 12 doesnt always follow the same guidelines.
I still havent figured out how to create a roster in Madden 08 and have it load up as a new franchise. It also crashes while loading. The only workaround is what ReeshMD posted earlier by modding an existing save.
Texture editing seems challenging too. I'm not 100% sure how to easily export/edit the *.msh files that are compressed in the viv's. Console Texture Editor (which I believe was used to edit GTA PSP) can pull up graphics but it requires a ton of guessing and use of a hex editor to find the proper offsets, which is painstakingly annoying so I gave up.