Sorry for changing the answer. I did not think at first, but ActiveJDBC can support this requirement out of the box, no need to implement new code.
Many2Many annotation is placed on one end of a relationship. This means that if you have TABLE1, TABLE2 and TABLE3 such that TABLE1 has many to many with TABLE2 and it has many to many to TABLE2, you can put Many2Many annotations on models representing TABLE2 and TABLE3, while TABLE1 does not have to have any annotations.
In this example, I omitted the join tables for clarity.
You can take a look at this:
http://javalite.io/many_to_many_associations#overriding-associations
for explanation of the "one sidedness" of ActiveJDBC annotations.
Hope this helps,
cheers