Or go back to having a proper joined up railway mentality - the inefficiency is down to the setup not the fact that its a loco-hauled train. Go abroad and you'll see the locos off the various CNL/EN portions abroad don't sit around all day working for the return working, they head off on freight or passenger work and in most cases won't be seen going back the previous night. You'd have the locos ending up in a different country would be more of a stopper to such workings than privatisation over here, but apparently not...
There is also the lack of runaround loops at Euston and Glasgow which mean you need two locomotives tied up to recover the sleeper.
Push pull working gets you around that as the locomotive that brings the train in isn't trapped there for the length of time the train is there.
Ofcourse shifting the sleeper into the soon to be far less busy King's Cross does solve some of these issues, but without DBSOs this gets troublesome.
EDIT:
As to having multiple units giving poor ride quality or whatever, I do note that there are two vehicles in each component of the Lowland sleeper that are not sleeping carriages (The lounge and seated vehicle).
This means that the entire formation would have four powered vehicles which could easily have 3200hp, which is probably sufficient to propel the entire formation, although as it might have difficulty with Shap should probably run on the ECML.
This would also require distributed transformers in all four power cars and would allow the actual sleeper carriages to be push pull wired conventional hauled stock (apart from one driving vehicle placed on the opposite end of the train)