I am aware that at a time Night Riviera Class 57s ran extra services with Mark 3 coaches. What sort of coaches were they (HST trailers or NR seated coaches?), and what services did they run?
For what it's worth, HST trailers can't be loco-hauled in normal passenger service except by an HST power car, as they use a different type of wiring & coupling to standard MK3 LHCS. They can of course be moved by normal locos using a translator or barrier vehicle, but that would be an empty stock move.