There's always a way to do something like that. Heck, just for fun, I had an old cheap Mustang 1 with the 10". I just took the leads from the amp and ran them to an input jack that I mounted into the ear panel with quick connectors. I ran it into a 1x12 I had. It was definitely an improvement, but the whole thing was just for fun.
Don't mean to be out of line, but man, with as nice an amp as you have in your Fillmore, my unsolicited input is to just do it the right way and acquire a proper 1x12 cab. Man, those things pop up on Craigslist, ebay, etc., for next to nothing.
In terms of what you have to look forward to, creating a 'mini-stack' (though it's not all that 'mini'!) with your combo sitting on top of a 1x12? You'll love it. My mini stack is a PRS "30", (el84s) 1x12 into an Avatar 1x12 w/ a Celestion v30 and man, that rig rocks! Big sound and great tone. It's especially nice if you're combining an open-back with a closed back.
Shoot, I'd LOVE to have a Fillmore 25 and do the same .
Good luck with whatever you do!