Ian,
I have been comtemplating this subject ever since you broached it. I just sold a ZBK-110 in .22 Hornet, so I am familiar with the action in question.
I would guess that what you have found that the caliber list ends with the 5.6x52R is correct. There is absolutely no way that the action in question could have a 12 ga. bbl fitted. (.410, perhaps, but that would be all.) The action is way too petite to have anything very large on it.
Are you planning on having the original barrel bored? If so, I would think that probably .25 caliber would be as large as one could go.
A .25 Hornet would be way cute. 
A 6x47 (6mm/.222 Rem Mag) should be doable. Or even a .257 Kimber (.25/.222 Rem Mag) Since I sold mine, I am not sure what the bbl diameter is at the breech. But if it's large enough for a 5.6x52R. I would think you could do .25/20 Win.
Of course another option would be anything on the .222 Rem case. (I believe these were originally chambered in .222 Rem) A more obscure option would be something on the .222 Rimmed case. (I just happen to have some new, unprimed brass....
) But finding a suitable supply of brass would be difficult. I would think you'd need at least 100 rds. before you started.
Just idle thoughts from a mostly idle mind....
Grasshopper 
