I haven't tried this, but you may be able to do this from .htaccess, *if* the domains are hosted on the same server under a subdirectory in the public_html (or equivalent) part of your directory tree. (It may be possible if hosting is on separate servers, but that gets into modifying a-records which I have no experience with.)
That would be my guess, although I've not messed with cnames, so I don't know the details. I'm just not sure *where* you'd put the .htaccess. I assume with acts similarly to a symbolic link. In that case you'd have to put the .htacess in a higher/top level directory than the sym link.
You said above that you'll make a tool that achieves this. I want to use your great product and this will be a great addition. Is it available in the new download?