Was that last line a pun?
I wish, but unfortunately in a FLA the path issue has existed since the beginning and is internally bound via you selecting the bitmap during import.
XFL looks promising, especially because even if they change the spec to include a full path you might be able to run a script that switches it to relative.
I share projects between different machines and yes, this is a wishlist item for me. You can wish for the relative path support too: