keith,
yes, this has been requested before. the answer, currently, is to call FF from ps, psp or whatever you're using and work with your images rendered in FF there.
FF could be expanded to include extra images at one time and it could be even done with layers, which would be great. the use of layers is what you really want, i think. imagine applying one filter to one layer in FF and then highlighting another layer and using another FF fitler on that. you could then export a merged copy back to ps or psp or export as layers. would be sweet
i forget what the FF team responded regarding all this, but you can look it up in the forums or maybe one of the FF team will respond. but i'm sure in favor of it!
craig
If wishes were horses... there'd be a whole lot of horse crap to clean up!
Craig