I would guess that Marriott won't give you a night credit for a room where you never showed for. They may bill your CC and you might be able to get points for the charge, but I bet they won't give you the night credit. Your best chance is to look at the various Marriott branded hotels around you and see what the cheapest rate is you can get. Usually, the lowest rates can be found at FFI and on a weekend night.

For the last Megabonus promotion, I also found myself a night short. I looked around and found that my company had a special weekend rate at a local CY for about $30. I jumped on it, checked in, but didn't stay the night. I drove back by the next morning and checked out and received my night and Megabonus points. ^