Your idea is interesting, but it only works on the following:

Planetouched PCs.
Characters with the Otherworldly feat.
20th level Monks (which are a myth).
20th level Incarnates.
Several PrCs.
People under the effect of a Shapechange spell and currently in the form of any Outsider.
People affected by the right Bestow Curse.
People under the effects of the Fiendform spell (SC).
People under the effects of that spell in Complete Mage that turns you into either a Barghest or a Yeth, I can't remember which.