With the clamp on mount instead of welded, it would allow longer use of the frame in terms of possible breakages. If the mount was to fail, the clamp could be replaced instead of having a potentially useless frame.
An engineer friend and I discussed this ; and the only possible solution we found was to use the BB shell itself as the inner part of the pivot ; then to wrap the outer part around it. Good Luck with the project!
A while ago I was trying to think up a full suspension fixed gear, and because of the movement between the axle and bb, I thought of a roller bearing around each end of the bb sheel, then an outer shell on the bearings attaching the rear triangle to front. The only issue I could think of was, the downtube and seattube would have to be really thin to accommodate the extra parts, which could result in complete failure of the join.