It is possible to get the handle of the RevoluteJoint and FixedJoint using the (python) bindings thanks to

https://github.com/robotology/idyntree/blob/ab9336447ab0c6d6d9f9d0c3d3a19be31140aed3/bindings/joints.i#L15-L41

It would be nice if these can be extended to other types of joints, in particular I would be like to have is/asPrismaticJoint