I made a proposal to WGT, which was basically the following:
If a PlayerA beats a player, PlayerB, who has a lower average, then PlayerA should get an "MPC Win: #", where # = PlayerB's average, on their ranked round score history. In other words, if a master/pro with a 70 average beats a legend with a 62 average, then they should have a "MPC Win: 62" tacked onto their score history, which would act just like a normal ranked round of 62.
The only possible complaint might be that it wouldn't help in this game's case since his average was actually lower than yours, in which case this method wouldn't have added the "MPC Win: 66.5" to his record, since 66.5 is not lower than his 65 AVG, but you could easily change it to Player B's AVG, minus five. If you did that, he would've gotten a 61.5 ranked round on his record from this match.
You could also weight this "minus five" part based upon how much credits were involved. If PlayerA wins a 100-credit match, then don't subtract anything, if he wins a 200-credit match, then use PlayerB's AVG -1, if he wins a 300-credit match, then use PlayerB's AVG -2, and so on, up to -5.
Of course, MPC players should also have their average go back up if they lose a game. It wouldn't be fair to a 70-AVG master that plays nothing but MPC's to eventually rack up enough wins against people with lower averages than himself and become a legend even though he's an honest 70-AVG master. In that case, you'd do the opposite: if PlayerA loses to a player, PlayerB, who has a higher average, then not only would, as described above, PlayerB have an "MPC Win: [PlayerA's AVG]" tacked onto his ranked round score history, but Player A would get an "MPC Loss: [PlayerB's AVG]" tacked onto his record.
Averages would then have a way to go both up and down, making sandbagging more difficult but also not punishing honest players.