Hmm.....
From the
PLUSBUS terms and conditions (.pdf download):
PLUSBUS tickets must be bought in conjunction with a train ticket and this ticket must have as its origin and / or destination the town for which PLUSBUS is purchased.
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PLUSBUS tickets are only issued for the town at which the rail journey originates and/or the town at which the rail journey ends. PLUSBUS tickets cannot be issued for intermediate towns along a rail route (that are not named on the accompanying train ticket).
The T&Cs do not explicitly exclude Rover and Ranger tickets, but those two terms quoted above would rather suggest you cannot purchase PLUSBUS in conjunction with a Rover or Ranger, as these rail ticket type do not have an origin and a destination printed on them.
That's not to say a ticket clerk wouldn't sell you one though.