Hi Grisell,
I understand that the casings could be a real problem.
Absolute "no-no". The only thing I cold think of would be to stuff cling film rolls, poach till firm, and then unwrap and fry afterwards.
As to what to use as a filling!!! I think there are some meat substitutes made from fungus such as quorn, but that contains albumen, so is "out" for vegans. Reconsituted TVP might work, possibly bound with lentil paste such as is used in vadais. Actually, maybe that kind of dish might be a functional equivalent. Things like felafel or vadais, which stick togther by being a puree, mixed with tasty chopped vegetables.
Not my tasse de thé, but then I'm a carnivore.