HI Bjorn,
Creating an instance of the /BOFU/BUSINESSPARTNER BO will never work, as the BO is designed to be a read only BO, and act as a wrapper to BUPA standard implementation.
that means that in order to create a new /BOFU/BUSINESSPARTNER instance, you need to use the BUPA relevant transaction.
regarding the general use case;
Lets assume you have BO A, which has an association to BO B.
there should not be a problem to navigate to BO B, and create it, as long as the BOPF configuration were done correctly, and the association is create enabled.
If you include BO B as a List UIBB, adding a create button to the list will add anew line which enables you to create the new BO.
(this is just one way to implement this, but there are many more options regarding on the actual UI design)
BR,
Matan.