Although there is a standing instruction that the Class 92 ETS must not be used so if the intention is to use these locomotives then the reason for the prohibition (which I don't know) will need to be addressed.
I'm also struggling to see how the reworked Class 73 is going to have sufficient power to work an 8 car sleeper over the Highland Line even without providing an ets supply. if there is any credence to this story at all then it seems to me they will need a pair.
According to this article -
http://www.class90electriclocogroup.co.uk/history_intro.html - there have been some 92 ETS tests in the recent past:
"I can report (at the time of writing in November 2013) several 92s have now been fitted with ETS equipment notable examples include 92031. This loco had its ETS tested back in August 2009, but failed on first attempt. A further test session ran in February 2010 using stock from Cargo-D which proved successful. However, the idea of using 92s on the sleepers to replace the 90s has gone quiet and is currently on the back burner."
....so I suspect that sorting out the 92 ETS issue isn't an insurmountable problem (they were designed to be able to haul the NightStar sleepers originally, which had high ETS loading).
But yes, I suspect the 73's on sleepers is/was someone floating an idea rather than a serious proposal. Serco might just be talking to GBRf about traction possibilties as a way of getting a decent deal out of DBS for hiring 90s + 67s...