MyFlashLabs There are pure JavaScript bundles for both OpenFL and Feathers UI. The only thing is that you'd need to write Feathers UI "externs" for Royale, so that it knows what classes/properties/methods are available.
On a related note, I created a proof-of-concept a while back that runs some basic parts of Adobe Flex 3 with OpenFL in a browser. It's using original AS3 code from Flex, with minor modifications for slight differences in OpenFL's implementation of the Flash APIs.
https://github.com/joshtynjala/openfl-adobe-flex-poc
There weren't any externs involved, since I could use playerglobal.swc normally, but it does show which compiler options are necessary to mix Royale generated JS with Haxe generated JS.