I get that you still have quite a bit to take care of for using the d20 system, and probably want to focus on that first. Just an idea for later consideration:
What about a Fudge system for Redwall? Attributes could include Str, Con, Int, Cha, Dex, Sensory, speed, craft, maybe a few others. Different creatures get automatic bonuses. Wouldn't necassarily have any advantage over D20 (as far as I can tell), but it would be interesting to try.