This was the advice given to me by the Diamond line:
Book a room with points for a friend or family member who can actually stay at the room.
You must book the room via the diamond line, and add your friend's name as a guest who will be checking in, and explain to the rep that you're doing a diamond challenge and need the stay, but you're unable to stay yourself.
A points-only stay still counts as a stay. It might even help to call the hotel once you're booked, just to confirm that they understand what's going on.

I completed my 8th and final diamond challenge stay using this strategy on Saturday, and my nephew and his wife got to enjoy the new Garden Inn near their home. However, when my nephew checked in, I immediately received a notice on my Amex card that a $175 charge was placed by the hotel. I called the Diamond line, and the rep said it looked like the stay was switched to revenue. I explained the above and he said it was not problem, and switched it back to a points stay.

(I had two multi-stay trips in March that got pushed back to May, effectively ruining my diamond challenge.)