CSR's in the past have informed me in the past that these seats may NEVER be returned to reward availability. It's not a risk I would take.
Some Options:
1) are there reward seats on the same or acceptable other flights now? Cal a different CSR and make another Reservation before canceling the first.
2) purchase the needed miles with the American Expres card, then after the owed miles hit your account, ask for a refund, and if one is not given, dispute the AMEX charges.
3) Have another one pass plat elite agree to get you the ticket with their miles, in exchange for you getting a ticket for them later on. Tell the CSR to split the reward with you "paying" for some and the other plat paying for the others.