Yup, not supported.
Sad day. I thought I could kick a bit more mxml out of app.
What would you want to kick out and why?
A pure ActionScript class is just so much cleaner- no namespaces, a constructor (instead of initialization handler), etc. It's a pretty minor preference, but it would certainly be cleaner and closer to my other components.
I think there may already be an enhancement request for this. I think one
of the reasons it hasn't happened yet is because we don't want to add
another pass to the AS compiler. The MXML compiler has a code-gen pass where
we figure out the dependencies so we can generate the styles you will need.
We don't want to burden the AS compiler in that way.
You can take the generated .as files and wire up the styles yourself, but it
is painstaking. It will almost always be easier to manage that from MXML.