WPMU - access site by subfolder and separate domain

I'm wondering if it's possible to run a Wordpress multisite (in subfolders) with something like domain mapping, but have the content still available though the original subfolder.

e.g.

domain.com/athletename/
athletename.com

both of these display the same content, but the athletename.com has an addition of rel="canonical" to the domain.com/athletename/

That way I can have all the content on the main site, while still allowing sub-sites to promote their own URL. Sorry if this has been asked before, I took a quick look and didn't find anything.

Thanks in advance!