Me thinks bmi would have thought about transatlantic from either Edinburgh or Glasgow at the same time as plans were put in place for Manchester.
A service out of GLA to BOS to replace the old service years ago by Northwest and GLA to IAD replacing United may well work.
I think you would generate traffic if the fares were right to fly UK airlines GLA/EDI to USA/Canada rather than Continental/American/Air Canada/Canada 3000/Air Transat