I GM both M&M and True20. Iv found a lot of Masterminds Manual optional rules very helpful In my true20 games. While I cant imagine some one who only plays true20 buying the book for those options since a good amount of it is about M&M powers.
What about taking the optional die rules, and other optional rules that would mesh well in True20. Cut away the power option section and make a PDF companion of the optional rules for true20.
After all the work is all done. Just cut away the M&M specific info in the work.