[Imp]

Andy most intresting proposition & if I understand correctly you have discounted using MP for facing changes rather are thinking on the lines of an exp check for facing changes.

A non turreted vehicle changing facing to opfire (or a turreted changing through say 120 degrees) would trigger a check for the other side to opfire so it may get off the first shot.
As Cross said could become gamey if all units get this chance but if its possible to just code so only the unit that triggered the turn gets the check it becomes a gun battle between those units.
This would certainly make a distinction between turreted & nonturreted vehicles/guns in that the moving unit might get off the first shot if a facing change is required.

Possibly even apply to other crewed weapons like MMGs as only one guy is watching out for the rear.