How common was it...

P-H the movie made a complete mockery of brave men that did take off in their outdated P-40's and P-36's and fought Japanese. Basicly movie is OK (and more or less accurate) up to the point when Aflleck and his buddie take off.

Back to the topic.
Soviet pilots oftenly removed part of the canopy. This is especcialy frequent on MiG-3 and LaGG-3 (early models). It is also ineteresting to note that becouse of problems with canopy Italians went back from buble canopy to open cockpit on Fiat G-50 and semi opened cockpit on Macchi Mc-200 Saetta.

A couple of pic's of planes with removed part of cockpit glasing:
MiG-3 (white 27) of 7. IAP near Leningrad in 1942 - airplane is also missing radio equipment and sports unusual blotch camouflage:


LaGG-3 ser.4 (white 14) of 44 IAP St.Lt. M. Evteev, Leningrad area in late 1941 aircraft is wearing a distinctive Nadia Bukhanova tiger patern applied in Zavod 153 in Novosibirsk:


LaGG-3 ser.3 of the 3 Gv.IAP of Red Banner Baltic Fleet winter of 1942/43 east of Lake Ladoga,

 

This sounds more like a fantasy because I can't really believe that anyone could have flown vast distances with a bad guy on his tail just so that a guy in the tower (in a forward based airfields, there wasn't even towers. Just a small container that acted as one) could shake it off with a gun. Neither does Rall's or Hartman's biographies tell anything of such a fight nor let alone hundreds of them.

Can you show us some links that describes an encounter what you described happening?