It's been an interesting debate and one I hope the club is looking at. I did the membership option this year where I think I paid for 6 games in advance, those got used pretty quickly but at the time that membership suited me. Anyway I've got no real skin in the game re the away allocation but my thoughts:-
1) If the club value the certainty of season ticket money in terms of budgeting then season ticket holders always need to be prioritised (as they are).
2) If available tickets for away games are less than the total number of season ticket holders then it would be fairer if there was a 'loyalty' scheme in place, to make sure that if as a season ticket holder you have been to 23 away games then you are more likely to get an away ticket than a season ticket holder who has been to no away games. (I appreciate the club don't normally financially benefit from away ticket sales).
3) Members deserve priority over non-members / non season ticket holders.
It's worth looking at what other clubs do, but my model would be:-
1) Season ticket holders start on 60 points. We've had 55 games this season, and I don't think any member or non season ticket holder (even if they have been to 55 games) should get priority over season ticket holders - if a non-season ticket holder ends up going to all home games then hopefully they see sense and get a season ticket next year! Season ticket holders then get an extra point for every away game ticket they purchase for themselves via OUFC ticket office. i.e. go to 23 away games, you end up on 83 points.
2) If you have prepaid for a Yellow Members+ (prepaid for 6 games I think?), then you start on 6 points. Yellow member+ get a point for every home league game from their 7th game onwards, and you get a point for every away game.
3) Everyone else gets 1 point per game attended if bought via oufc.
Then when we come to Posh away tickets they are made available to season ticket holders in order of priority, you'd need to look at who has what points but it might be something like 600 fans with 80+ points, 400 fans 70-80, 4000 with 60-70.
Above suggestion ensures that regular fans (but non season tickets / non members) get a better chance of securing a high demand ticket (if available) than the fan who just turns up for Arsenal etc.
I think you have to award points on a season on season basis, I say that as someone nearer 50 than 40. We need to give the youngsters a chance to get the bug and if you prioritised someone who had built up points over a number of years then you could end up blocking the chances of younger fans securing in demand away tickets. For me as a youngster it was the away days that got me hooked, the days out and adventures with your mates etc.
Some random thoughts:-
** The club may want to offer 2 or 3 points for an away game that they financially benefited, for a game that would not sell out. For example a 1st round FAC game at Oldham would not sell out, but OUFC would financially benefit with every additional sale. (Note: appreciate I may be overcomplicating it).
** It's worth trialling things soon, then we'll be in a position to know what works if the Posh away situation becomes a more regular occurrence.
** Next year if I buy a membership (as opposed to season ticket) then I'll be buying one for my daughter. A membership for her would have been helpful when it came to booking my Posh home ticket.
** I wasn't a big fan of season ticket holders getting the chance to buy a friend a ticket for the home game before they went on general sale. Fair enough if that was an advertised benefit of season tickets but I don't think it should be. If season ticket holders took up that option then I am sure most would have used it for Oxford fans, but also I'm sure some will have bought it for a non Oxford mate who fancies going to a big game.
You could tweak a loyalty scheme in so many ways, and I think we need a revamped one next season that includes the chance to differentiate between season ticket holders.