Regarding class A, that is a powerhog But i dont think the crossoverdistortion in a class B bufferamp will be any worse than the stepdistortion from any cellphonefriendly D/A converter.

Anyway, limiting the output could be done by limiting the swing for 0dBFS too, thereby making it possible to optimize the electronics for any kind of capacitive/inductive output that our cables (and headphones) can present for the amp.
-Atleast i will always have somekind of preamp with variable gain to plug the output into

However i still dont think it is useful to do anykind of "earprotection" when headphones dont have a standardized sensitivity.