I like the mechanic personally, and I think it works for non-combat as well. That is, DCs run from Super Easy 10 to Super Hard -10, with 0 being bog standard. While I realize that ascending AC is somewhat intuitive, using Target 20 is also intuitive so long as you envision it as a penalty on the attacker's roll, not an inherent quality of the character.
Unfortunately, I do seem to recall that 2e best AC was actually -22 or something like that, once you tacked on all possible bonuses. (At least as best as my friends and I could figure.) However, that required a bunch of buffs like Invisibility and quite unlikely stats and magic items, IIRC. So maybe the ranges should be adapted, or maybe not. I suppose that would depend on the design of the new system (stacking rules for one easy example.)
All that being said, while I would support such a rule, I'm not holding my breath for it. For whatever reason, too many people find minus signs scary.