Can an Extras package upgrade a Core one?
Preferably not. :-)
Or maybe split the process in two and submit a new version of cdrdao to Core (how?)
Look at the Red Hat Bugzilla -- http://bugzilla.redhat.com/ and see who the maintainer of cdrdao is. You can offer to take over the job for Fedora, or you can submit the package to that person.
and cdrdao-gcdmaster in Extras only?
That's probably what will happen.
Unless you want to move both to Fedora Extras. I think that wouldn't be such a bad idea, since we already have cdparanoia in Core.
-Max