^See Unearthly Weapon Specialization.
Or are you looking for an excuse to get a vorpal attack on 100% of your attacks? If that's the case, why not just make a Trans. that says you instantly kill anything that isn't immune to crits? (And yes, that's overpowered, but...at that point, most melees will be instant-killing 90% of their enemies with one blow, anyway.)
Either way, I wouldn't allow such an ability, since, taken with UWS, you're looking at a cubic increase in damage that takes the numbers from "absurd" to...honestly, "stupid."
For example...oh, I don't know,
A Colossal First One wielding with 410 strength wielding a +300 Neutronium Greataxe.
So...922d10 +648 20/x3
Add the usual ability suite and we get
922d10 +658 14-20/x11
Add UWS and we get
101d1000 +7238 14-20/x11
Which, maximized, gives us 108,238 damage a hit. (and that's ignoring Power Attack which would factor a hell of a lot more damage onto that.) with a 35% chance to deal 1,190,618 damage.
Add your proposed ability and we get the crit damage on every hit. :|
Oh, and that's ignoring Uncanny Weapon Spec., since that would give us an even more stupidly outrageous number. Let's see...colossal greataxe does 4d12, doubled twelves times is...roughly 20,000d10 before adding UWS.
Bottom line: melee does not need anymore help.