My old Aprilia 650 had a cast muffler hanger that mounted approximately by the rearsets, and followed the exhaust pipe to finally support the mufflers. It wasn't the lightest, but it did work. Your bike would look really incredible without any muffler supports from the subframe.
Alternatively, some kind of compromise. For instance, make a small aluminum truss of 3 small rods and cross-braces to go from behind the rear-sets to the muffler, then a really small rod from above (assuming that adding the truss would let you get away with a much smaller support that what's there now). On the other side you could integrate some kind of chainguard.