Pay with points
Dont you have the option in Amex MR to pay a cash hotel charge to your card with points (1p/cent or 1p/1.25 cents depending on your card)?
Make sure it makes sense to transfer to SPG points to pay for the room rather than using your MR to pay for it directly.
Also, I believe when using your MR points directly, you would earn SPG points on the reservation.
--B3