The only afermarket belly mower I've seen that would fit was run off the rear PTO. Not really the ideal set-up. Seems I recall seeing a Kubota belly mower on CL recently. I never look outside New England, so maybe a search my turn up the same mower for you. If my recollection is right, it fit the 7800, 2710, and 2910....... though I'm not sure if it was the 60 or 72 inch.
I've always got my eyes open for a used 72" belly mower for my 3030 also, but have yet to find one that isn't priced 'as new'.