I have to say that it would never have occurred to me to assert a pre or post condition and an expected failure where I expected the pre or post condition to fail, but if you've got a real use case and it would make the code simpler, I suppose I have no serious objection.  I don't use expected failure myself.  Just make sure you document it well, since it is not a behavior I would expect when using expected failure, and I'm sure there will be others like me.