Hi Daniel,
This is a good idea for customer to extend field in custom BO, I was working on solution where we have thought to provide key user field for custom BO but it seems not possible in current version
and I came yo know that SAP key user fields created in adapt mode sits at a different table in backend, and partner content also have different place in repository.
Currently we can create key user field for SAP BOs. I am also interested in this feature and wish that SAP will provide this feature soon.
Regards
sunil