I have alway wondered if the bow straffing guns could be implamented with an opening door much like the mods we have for some planes where the rear gun canopy opens when enemys are near or since the PBY has no opening canopy, could the bow doors open using the key to open canopy.
That would have been my first port of call - but I did a little research into the cannon installation, and I found they were mounted like this:


If I do a version with a heavier weapon however, i'll do as you suggest; the bow door will open and the gun will slide out when the open canopy key is pressed.
For the historically accurate 20mm cannon, i'm not entirely sure yet as to how to proceed - it'll either be a remote turret with limited traverse, with the guns being controlled from a now weaponless front turret, or i'll install a fixed reflector gunsight in the catalina cockpit.
barrett, i'll say now to avoid getting your hopes up - I won't be making the PBY-4 version. If you can produce the relevant 3D parts however I will generate a new slot for you.
-Matt
Edit: here's another gun installation which saw service:

4 .50 caliber brownings mounted in the bow door. This site is worth a look:
http://www.daveswarbirds.com/blackcat/photo4.htm