Actually thinking it through it should work because although the biggest bang for your buck is using your MAR Visa at Marrriott resorts I know not everyone does and I'm sure they get their points or cash back or whatever that alternate card offers as a perk so in this case the SPG Amex would get the offer as proposed without any hook to the MAR points you get for just staying at a Marriott and referencing your MR account#.
Sorry, unclear on your comments. The question is how to get the most MRPs, including converted SPG points to MRPs. Is the answer to that question to use SPG Amex to pay for Marriott incidentals or hotel stays, and add MRP number to reservation? I have both cards, and until the merge, I had always used Marriott Visa for Marriott stays.