You've definitely tried imperial Allen keys? My Bob Jackson from that era had an imperial bolt. I think it was 3/16" which is between 4.5mm and 5mm, so no metric hex key will fit.
Looking at that problem, and assuming the plusgas and oversize keys have been tried I'd use a 1mm cutting disk. Even if you loose a tiny bit of the clamp material it won't matter too much IMHO.
On all my frames bar my Pinarello, which has a threaded seat lug, the bolts are the Campag type so if it were cut it would come out in two pieces on either side of the seat cluster.
On all my frames bar my Pinarello, which has a threaded seat lug, the bolts are the Campag type so if it were cut it would come out in two pieces on either side of the seat cluster.