This is what I plan to do:
Basically, I request a custom account with my webhost that has around 10mb space and 10mb bandwidth and lots of add-on domains. I register domains on behalf of my customers, and add each on to a separate directory. I create the same directory on my ISP webspace (with unlimited bandwidth), and use the apache mod_rewrite function to send all files requested from the newly added domains to the ISP hosting. Then I have a login on my ISP pages where my customers would be able to log in and upload files to their directory in the ISP hosting.
Theoretically, I think theirdomain.com/some.doc should go to ISPhosting.com/~user/theirdomain/some.doc.
I can't figure out how to make the redirects to be transparent. If there's an alternative solution I'd be happy to hear it. :)