Can a turnpad also be used as a runway holding position?
Not withstanding Simon's answer, the answer would be that it depends on the runway. The runway in the picture above has a displaced threshold and what appears to be a hold short (runway exit) line separating the runway from the turn pad.
The hold short line would indicate that the turn pad can be used as a place to hold while waiting for clearance for departure. With or without the displaced threshold, ATC could clear you to backtaxi onto the runway in order to reach the turn pad. Once there, you would begin holding short once the entire plane is beyond the hold short (runway exit) line. Holding there would not be an impediment to either arriving nor departing aircraft.
For departing aircraft, ATC could use the turn pad as extra room when having multiple aircraft performing Line Up And Wait maneuvers, preparing for departure. A scenario with a displaced threshold would put aircraft on the turn pad far away from the start of the Landing Distance Available.
Without a hold short line, ATC would not be able to hold the aircraft on the turn pad. Although, they could still issue a LUAW instruction by having the aircraft back taxi on the runway. It would probably then be up to the pilots discretion on where to perform a 180 turn around and where to hold as long as it is before the AER demarcating the beginning of the LDA.