Unfortunately I fear any Aberdonian service would be running via the WCML to avoid the need for extra route knowlage. Unless it was included as part of the EC franchise.
I'm pretty certain that the Scottish sleeper crews have ECML route knowledge anyway, as its often a planned diversion.
I'm not entirely sure how many SLEPs there are. If there are enough of them, then form up enough for the entire Aberdeen train, with the ability to leave some behind for off-season/midweek workings if necessary. Add some ordinary MkIII FOs if there are any spare, plus a Chiltern-style DBGV (heavily silenced). The set could then sit in a platform without using too much power. Ideally, as I say, it would operate from King's Cross Platform 0, with calls at Peterborough, York and Newcastle, then take over the current Aberdeen working. This would make it possible to strengthen the existing Fort William and Inverness portions, or to run an Oban portion if there is demand.
AIUI, Transport Scotland want a separate sleeper franchise. I'm not sure whether this will involve new or refitted stock, Chiltern-style conversions are possible. A bit of growth to the franchise would be ideal. I'm not sure what the current capacity limits are, but the fact that it has to work out of two platforms at Euston would seem to be one of them.
Theres definitely plenty of SLEPs around still, quite how many of them could be easily returned to service is a different question though. Many of them seem to be in quite a state.
I'm inclined to think there are a reasonable stock of Mark 3 FOs available, even if it means buying them from charter operators. Of course there are plenty of Mark 3 DVTs which cover off the lack of Mark 3 brake vehicles.
I think before too long (particularly if the current 2 each way services are split into 3) the sleepers will be operating from Kings Cross anyway. Euston is a difficult prospect while HS2 works are taking place, and Kings Cross is going to be rather empty before too long. Fits nicely.
Chiltern style conversions would be all well and good, except that the power doors wont really fit on the 'cabin' side of the stock. There was some discussion of this on a recent FGW sleeper thread, given the accessibility issues once onboard the stock I wonder whether power doors on the 'corridor' side, with the SLEPs arranged so that alternative coaches on each side have power doors, would be workable.
One aspect that I think would work exceedingly well is the inter coach doors which separate the corridor connection from the main coach. Given the complaints about noise, these could make a significant difference to the passenger experience.