hazed, about a month ago, Funked, mietla and I were checking out the 88 at the USAF museum. The problem isn't whether there'd be room for the bombardier. The problem is that running a 20mm cannon would block the gunsight. fishu's interior picture might be conclusive, but I cannot make out either gun or bombsight down there.
Actually, come to think of it, it would explain a lot of things.
You couldn't fit a 151/20 in there. The barrel is just too long. Right behind where you see the barrel is the bombisght. So a rule for mounting a gun up there is that it cannot extend back into the body for more than a few cm.
But the FF has a really short barrel and would probably be the biggest thing you could fit on there. I think it possible that it was stuck on there with the bombsight working.
In fact, compare Fishu's ju88-2 picture with mietla's shot of the MG/FF here:

.
I have a figure of 53 inches (134 cm) for the MG/FF (not barrel, the whole darn thing). If ju88-2 shows all but the very back part of the gun projecting from the plane. They could have crammed it in there.
OTOH, Ju88-2 seems to show that the bombsight was painted over, implying funked's account.
[This message has been edited by Dinger (edited 04-02-2001).]