This is an odd case, that I don't see us supporting anytime because (a) this is a unique problem to a very specific file storage site (b) DropBox isn't typically used to host "web server files" (that is, files that are required by your website to function -- you wouldn't host your images on DropBox either), and (c) this would add a massive amount of manual work for you if we were to allow you to easily make arbitrary URLs for every *.wyu file.
You can always manually alter the *.wys file to correctly reference the arbitrary URLs, but that seems like a big hassle. Why not just use a dedicated host?