Thanks very much chaps, and especially Steve. I do have the Vasco books mentioned, although there is no mention of the spent case ejector method with either the 'Dackelbauch' or the ETC rack in either book.
I've read varying accounts and specs regarding the 'D', and the fitting of the 'Dackelbauch' has been described variously as D-O and D-1/R1, with some descriptions stating it was able to be jettisoned - unlikely, I think, due to a) the method of attachment; b) it's immense size would almost certainly be a problem, and would/could foul the fuselage with disasterous results: c) those aircraft attacked over the North Sea on August 15th, 1940, jettisoned their wing tanks, but not the 'Dackelbauch'.
Accounts by former members of I/ZG76 don't mention the cannon, or jettisoning the belly tank, although mention is made of the terrible handling 'qualities', especially when the tank was almost empty, with fuel sloshing around, and the fire/explosion hazard.
Given that cannon could be used with the ETC rack in place, I'm beginning to think that perhaps there was a 'catch tank' for the spent cases with the big belly tank, although given the limited space, fouling of the ejector ports could be a problem.