They already allow the use of miles to pay for wifi (also auxiliary revenue? – but granted, probably makes less money than cash for E+). They could easily tie the cost of the redemption to the cash price of the seat. I imagine the bigger hurdle is dealing with seat changes, equipment swaps, etc. The system handles that for cash purchases, but I can see updating the system to support miles as the currency for all of those scenarios would be non-trivial.