Instead of using an association to get the related objects implicitly, you should use "Expand GRM Relations For Primary". This will give you access to the relationshipObject which has an association to the otherSideObject and the _relation (ImanRelation). Your custom relation is derived from ImanRelation, so you will need to create a similar domain model in Mendix to get this data mapped.
I haven't actually tried this, but this is my suggestion