The problem is, what's the purpose?
Transport planes are of limited usefulness in Special Events (in FSO base capture has been VERY rarely implemented, and is generally a GV-only affair in which aircraft are not allowed to participate). MAYBE they would get some use in the AvA, but it's not like people are breaking in the doors to get in there.
In the Mains the Ju-52 would be even MORE vulnerable than the C-47. She was slower, and the gun would be of virtually no use whatsoever (it's a single 7.7mm. Ask B5N pilots how much of a deterrent that is).
This puts other transport aircraft about as low on the priority scale as you can go, ranking only a few percentage points above the P-63....