The only place you could obtain an excess before boarding would be the ticket office (the passenger already holds a Standard All Line Rover).
The iKB has exactly the same text for "opportunity to excess before boarding" and "no opportunity to excess before boarding" suggesting no penalty for the former.
I do note that the public page on upgrading to First Class (
http://www.nationalrail.co.uk/times_fares/ticket_types/46557.aspx) is much harsher in tone than the iKB (says not possible with BritRail pass whereas iKB says same procedure as with a Rover or Ranger). There is no public equivalent of the iKB excess procedures pages that I can find sadly.