St. Pancras would be the perfect London terminus for the Sleeper, let alone Euston. Direct electrified links to the ECML and WCML without need to revers, sufficient size of platforms, greatly enhanced European rail links to and from Scotland, greater links to Kent and the South Coast.
Given that Platform 5 is the obvious choice, what security measures are you potentially looking at being put in place?
Obvious temporary partitions on the edge of platform 6 and the moving walkway leading to the departure lounge, as well as relevant ticketing checks at the main gates would be essentials - even then at first thought it seems perfectly possible on an operational and financial sense. It would seem more feasible than running 6 rakes back and forward the ECML a night I'd have thought.