An "old" Red Dragon, with CR 20, has a breath attack that deals 16d10, or averaging between 80 and 100 damage per use. On the rounds when a dragon can't use its breath weapon, this dragons Claw, Bite, Wing, Crush and Tail attacks are split 8 ways, plenty of opportunities for DR to take effect; assuming I'm reading my tables right of course.
While I agree that armor needed a buff, and your system is very original, I'm not sure it is balanced against most monsters. The only time I could see this really being a worthwhile change is if you are going up against an enemy spellcaster, who can cast the equivalent of a dragon's breath attack every round, and even then the numbers seem a little high.
Maybe we could have something similar as an option for magically enhanced armor; and leave the normal AC/DR in place for the rest of it.
The other thing is, while this certainly evens out the disparity between fast-multi attacks and big-single ones, I can't help but worry if that will take some of the excitement out of games. To much tuning turns every encounter into a math battle, where the side with the bigger numbers wins, and strategy, tactics, and fun go out the window. It's perfectly possible I'm overstating the issue, though, and if you like this method of doing things, than I'd be happy to see a full write up.