No offence intended but I'm not sure simply insisting you have a "FA" on the flight deck to prevent sole occupancy of the same is the answer here.
Certainly on a 2 pilot long haul flight as I'm sure you really know we're not just leaving the flight deck to "parade"
, there are genuine physiological reasons that might require absence from the flight deck for slightly longer than the time needed for a quick P break. In some circumstances there's going to be a possible conflict between the need to keep the galleys/passenger cabins monitored if there's also going to be a requirement for two crew members to be on the flight deck at all times .....
Of course I'm sure any manager worth his or her salt would be willing to budget for the extra manpower needed to provide the obvious solution to the above problem.