Hi Oliver,
it took a while, but we finally understood how to implement this (btw: the BOPF training was very helpful for this :-))
We created a link table and own associations between the link table and both nodes. Then, additional associations can be implemented for navigation between the nodes directly, internally using the link table. The direct associations can also be used in the OData Model using SADL binding. Hooray!
Thanks again for pushing us in the right direction!
Svenja