Originally Posted by Jon Maiman

You were lucky last time. FNA's only work with standard (or pointsaver) redemptions. FNA's can't be used for any award stay with upgrade (neither cash nor points). I ran into this issue two years ago. In my case my FNA was worth more than the total point cost for the desired redemption with the point upgrade. I thought it was a website bug and called MR customer service. They explained the restriction to me. Wound up booking an alternate property instead. So unless the rules have changed recently, you won't be able to use your FNA for your MKH stay in a Beach Front Room.

My desired stay was for a LHR airport hotel, so it was easy to just select another MR property instead. One possible workaround, never tried it, would be to book a cancellable base room at MKH if one is available as a standard redemption with your FNA + topoff if needed. After doing so, call MKH directly and see if they are willing to offer you a cash upgrade to beach front. This would definitely be skirting the system. I have no idea if it would work. YMMV...

--Jon