Rather than diesel-hauled, if sent along the DC line was this provided by one of the Bletchley dmus which did various turns, including the St Albans branch, in those times? I seem to recollect reading something about such an operation.
Although I mentioned the 03:50 as being a Staff Train, it seems from old WTTs that the train also carried newspapers and parcels for stations along the southern WCML (must have been the last newspaper departure of the night). It was booked for quite long station stops at places like Watford Jn (8½ mins), Hemel Hempstead (14 mins), Berkhamsted (4 mins) and Leighton Buzzard (2½ mins), and was formed of BSK+BG+BG. Probably wouldn't have had space in a DMU for all the newspaper bundles and parcels.
Having said that, there were other staff trains running between Euston and Watford in the early hours. These were booked for a DMU full time - not just during disruption - and these were highly likely BY's diesel unit off the St Albans branch. This would have made them easy candidates for DC Line diversions when necessary, or just handy to keeping running on the main lines if some of the the fizzy knitting was switched off during the night.
For example, in 1977 there was 2A00 01:00 Watford Jn - Euston DMU (unadvertised), returning as 2A01 01:49 Euston - Watford (also unadvertised)
Also a 2A03 02:40 Euston - Bletchley (unadvertised EMU, which had arrived with the last southbound 310, the 00:15 ex-New Street).
Several of these late night runs were booked for unadvertised (to the public) calls at Queen's Park main line platforms - but that's OT for a thread about the connection at Watford Jn.