I've read again your post and maybe it's my fault, not the adobe forums one
I've never tried something like you're asking about, I don't have clear in mind what you
want to achive. If you want only a "core" of classes well, you can develop/write them
using Flex Builder but if this won't include components than what's the point of compiling an swf? I mean, you can write those classes and then reuse them into flash
only importing them..
Again, having not clear the final result I don't know if my answer is correct or not