I hate olives and usually just leave them out - and from your list of ingredients I think the cassoulet will have enough flavor to not need the olives or a replacement to be a delicious meal. 

you're looking to replace the saltiness of the olives so how about some diced parma ham or something similar.

try and get the ham in a chunk or thick slice and dice it yourself, i live in spain and this is quite typical in dishes with beans and lentils.

alternatively use chorizos, either slice them and cook at the same time as your sozzies or if you can find a slightly firmer cured type then dice them as with the ham.

In the end I left the olives in then removed them after cooking, the dish was very tasty but a little heavy on the garlic - I used sicillian sausage, thanks muchly for the input though and I will try the suggestions next time I cook it

Simon