Huh?
A PPL holder flying a civil registered aeroplane for the purpose of non-private flying?
This sounds awfully close to being illegal, if not actually so, as you would be receiving the valued consideration of free flying.
If there aren't sufficient military pilots available to meet the 'task' requirements, then the pilot requirement should be advertised. In my opinion such pilots should hold CPLs for this work.
Sorry to sound like a trouble maker, but I wouldn't like to be in your shoes were something to go wrong.
Or were you referring to VGS flying?