I doubt DRS can enter into another contract to supply another 4 or 5 brand new loco's TBH plus they need them for their own use.
The real biggie though is that their use on the beds would push it over the edge into being completely unviable.
It makes no money now as it is! 68's also have higher track access charge versus 57.
There is also the rather large problem of lots of GWR drivers being trained to drive them, this would also be very, very expensive unless you use drivers from another TOC in which case you would see GWR drivers on the Night Rivera link out of work!! = very bad news!!
In short, 68's on the Night Rivera- its never going to happen!!