I am certain it did happen because I noticed the working when something like a Special Notice (SN) came across my desk, I imagine I was doing a cross-boundary working to the SR that was contained in the same SN. I have a feeling it was for the Central and / or the SE division as I didn't know the geography very well.That might be technically possible, but I haven't seen much evidence of it actually taking place, especially on a regular turn. The series of newspaper/van trains run by the SR out of Victoria/Waterloo in the early hours (0200-0300 departures more or less) were essentially parcels-type stock with mostly a single BSK in the formation.
Special Notices were on A4 sheets (or multiple sheets) combining loco, stock and traincrew workings that were issued for alterations that were too late to be included in the Weekly alterations [to the WTT.]
I suppose it is possible that engineering work had changed their usual arrangements. Like @Taunton, I think it would have been easy to do, you just had to have the vans and coaches marshalled as the tail behind the last MU-fitted loco.