We were doing this, but it's a hassle for the person collecting payments (at the time, it was yours truly). So each person now as their own mycuse account and renews their ticket. We're trying to add 1 next to us this year, but I'd be shocked if that worked out. We're front row, on an aisle. Limits the options of where the 7th person can sit.
As to the original post, I'm surprised the Dome won't do it. But I'd guess that if you release the 2 seats, the odds are pretty good that other 2 could then 'upgrade'. It would just be 2 transactions and there's the possibility that someone could swoop in and grab the 2 being vacated. If you time it right, and let's be honest, the demand for SU tickets isn't all that great, you should be able to pull it off.