And this is why it would be "nice" if Flash had some way to do multi-threading, or something to make it easier..
I suspect (?) you're out of luck, you'd probably have to dig deep into the source and customize it to act as a 'pseudo thread' (if even feasible). Probably not worth the level of work it'd require: ex. http://blog.sharendipity.com/simple-actionscript-pseudo-threads