If the OP sees desirable space now, why not book a RT ticket now or a one way for the price of a RT? As a DM or PM, there's no cost to cancel it and redeposit the miles unless you want to use the miles for something else in the meantime. I don't think that having booked and cancelled the ticket, in the event that you can't turn it into a one way, will significantly affect the chances of getting those same seats as a low one way later. However, YMMV; this is just a guess.