Modern Elmo... at least a year ago, there was a discussion on the MV-22's exhaust... the heating only became a problem when its engines were run at idle for more than 20 minutes or so in the same spot, or at full power for over 5 minutes in the same spot.
Initially, time restrictions for "same-spot running" were put in place, and portable "hot pads" used to protect the deck, but a permanent solution was soon developed.
The majority of the "fix" was to simply not run them for so long at the same angle (it was determined that adjusting the nacelles' angle a few degrees every few minutes greatly reduced the heating effect), and the rest was a new heat-resistant deck coating!
The MV-22 has been operationally deploying shipboard for at least a year now, with no noteworthy deck-heating issues.
Widger... "chained-down hull-power runs" on an F-35B will be conducted the same way they are for all other jet-powered aircraft on normal carriers... with the tail at (or just over) the deck-edge, pointing outboard!
The nozzle will only be directed down during these runs to verify correct functioning of the actuators, there will be no need to run them for any significant length of time while pointing down. This should be relatively simple to do with the tail extending past the flight deck... thus no worries about high-output exhaust blasting the deck coating or splashing out to injure nearby crewmen.
No worries about heat from the lift-fan... it blows unheated air!