Hi Marc..( crankcase ) ,. Photos showing the inside of either pilot or gunners positions are almost non-existant ...There is one overhead view of an early V type U/C machine for photo reconaicance work that's quite good , but there's not much more .
Structuraly , there is no reason why (Lap ) seat or any other belts, can't be fitted , though they would not be much use in combat , as also the gunners seat , which , although made and supplied , has been discarded from this RAFM machine since delivery as it supposedly interfeared with the front MG pillar tube .
It is posible that the gunner might have worn the full " Safty " harness that actually "was" available at the time . This incorporated a strap , hanging down between the gunners legs , and presumably adjustable , to be fitted by quick release fitting , to a eye bolt in the floor of the front cockpit ( Or anywhere else that might have proved more suitable ) . However , this would probably have been in most cases , impractical , in heat of combat ;
Although procarious to stand to fire back over the top wing , it was not realy any safer to just sit down and get " Shot -up " !..A case of which was more for self preservation under the circumstances .
At safer times on the way home , when the Lewis could be clipped to one of the side steady clips ,; The gunner / bomb aimer could raise and lock the "pull-up " fabric windscreen ahead of him , which offered a little sheilding from the biting80 mph gale . Also he would have been able to do Camera or aiming work down in the cockpit , behind this in the right circumstances .
Agread ,being a front gunner /observer (and he , usually the officer ) was not the " Best " job in the world , but then neither was being in the trenches !
Best regards John.

PS ..Anyone with any "contemporary" photos of either cockpits , or " Under the skin " details , would be most welcomed to show them here .