I'd like that too, tho ideally we'd also need code for him to get up and man the gun when he sees a threat (not just is ordered to).
It may be possible if OBD would be willing to incorporate a similar functionality to what the OP is requesting ...
For example, assign a key to toggle between inactive/active observer. Inactive Observer will change him into a sitting position and no longer use the weapon. So, the observer would need to be "ordered" by the pilot to sit and stand.
OBD have done much more complex coding (e.g. recon wars), so I wouldn't imagine this to be too complicated for them. Maybe OBD can "re-use" the sitting observer model to switch between sitting and standing.
... Although, it would be a bonus if they could code the observer to "automatically" stand if he sees a threat.