Thank you - confirmed. I just dug up a copy of Flash CS6 and tried this:

  var val : String = "foo";
  var foo : XML = <foo><bar prop={val} /></foo>;

...and it worked. I swear this wasn't documented anywhere. That said, I just googled a few Flex examples (I never used Flex) and it seems to be common there, so I guess that explains why at least someone knew it existed!