No you cant do that anymore. The new fares system does not let you make up fares in this way. If it is not in the database then you cannot sell it.
That was the impression I got when I spoke the clerk in Warminster - if it didn't exist, then he couldn't sell it !
My advice would be to sit in first class and then ask nicely for an up grade when the TM▸ comes around. When they find that there isn't one they will probably let you sit there anyway.
To be honest, that was the thought in the back of my mind. If the station couldn't sell me a ticket, I couldn't really see how the Ticket Inspector could. Would the "system" let him impose a Penalty Fare without issuing some sort of ticket with it ? There must be a paper trail with this, otherwise penalties could be issued, literally, for nothing - or disgruntled passengers would certainly claim that !
The very worst I could see happening, was being asked/told to move.
I will still ask at Westbury before I buy the tickets though!