Mate,
I do not think it is possible (IMHO), at least not while in the air. The pilot would have to somehow place the rudder stick between his knees, keep his feet on the rudder bar, somehow reach under and over the cowling to pulll the belt along and feed it into the acceptance slot and then somehow cock the weapon several times to advance the belt. Dump the rest of the ammo belt into the ammo canaster, so that it would feed right and still fly the plane in level flight for quuite a few minutes. I would say that it would be a rare feat with someone with 4 arms and eyes in the back of his head to say the least.
If it was possible, I sure would like to know, cus' I think I read about it somewhere in a book, but the book could have been fiction as far as my memory goes.
and before I forget:
Welcome to the flagship of the Aerodrome - where the greatest minds on the planet get together and talk about their favorite subject - WW1.
ttfn
tcrean7828
tom
P.S.
Werner Voss fan here.