I did this during my own Plat challenge, when i had no status with Marriott. This was done in two cities; in one there is a property with 4 hotels all on the same lot.

Initially, my very patient wife who was on a business trip with me hopped a night at each hotel, doing the following:

Night 1 - Stayed at Hotel #1
Night 2 - Moved to Hotel #2 (stayed), but also booked a room and checked into Hotel #3
Night 3 - Moved to Hotel #4 but also booked a room at Hotel #1

In all cases I avoided a duplicate night stay at each location, and yes, I did physically go to each hotel (they all share the same parking lot), and checked into the room, went into it, and tossed around the sheets. There have been reports that even if you check in and there is no visible stay by you (i.e., no room card usage or signs of using the room) that your stay may be removed. The next morning I checked out individually of each hotel, packed up, and switched. I did actually take advantage of the free breakfast at the other hotels, since the ones we stayed in didn't have any (and I had no status).

In all, 5 nights, 5 stays, all individually posted. Each were fully paid stays - not a 1/2 night or day rate, but yes, I did book two separate full rate rooms on the same night, as the calculation of Plat benefits was worthwhile to me.

I then did the same in another city, where there were no side-by-side hotels and did a fair amount of driving, but to give my wife a break we stayed in one hotel for 5 nights, and I just booked a room at another for every other day.

As soon as I reached 9 stays, I emailed the plat challenge, they reviewed my stays (with all the duplicate nights), and I was given Plat status.