Okay, I see. I misinterpreted the intention for redeeming the reward.

In that case, I would consider printing the ticket and changing it at the station for the actual travel date, when known. (This presumes that the validity date for the ticket is ~1 year from travel date, as for paid tickets. Someone correct me if reward tickets don't have the same validity period.)

I know this goes against the advice of earlier posters, but printed tickets have value, and it's easier than it used to be to make changes to printed reward tickets at the station.

nerd's original idea is interesting. Book a reward, wait for AGR to zero out points, then redeposit reward. Clever. Would be an interesting loophole if this actually works. And knowing AGR, said loophole probably wouldn't last long if enough people use it.