You said:
what I am interested in is to list all m:n relations, and thats it
Then you said:
if the m:n relation navigator in the LLBGen was not read-only then it would solve the case but if not then it is useless for my app
I'm not sure what do you need exactly.
You can fetch the Bs by calling a normal fetch with the appropriate filter.
e.g.
SELECT * FROM B
WHERE B.ID IN (SELECT BID FROM AB WHERE AID = xx)