All, I believe that I've found a way of getting around Marriott's IT issue where SNA's cannot be used on advance purchase rates. It takes advantage of another error in the Marriott IT system (go figure) where changes in different windows are not handled properly. It seems to work for me - would be great if someone else can confirm.

For this to work, in addition to a pre-paid reservation where SNAs are disabled you need a rate at a hotel where SNA's are eligible - so go book a bog-standard fully flex rate at your local Westin or whatever which you can cancel later if you don't currently have any flexible rates booked.

Log in to Marriott.com and click on "My Trips" (obviously this won't work for people whose trips are still not showing). Open "View/ Modify" in a new browser window/ tab on the flexible reservation first, and then open "View/ Modify" in the non-refundable reservation in another new window/ tab. Go back to the flexible reservation and click on "Start Upgrade Request" and it should take you to a webpage for the pre-paid reservation where you can finally select your SNAs.