Hi Bjorn,
your are right: Cross BO Associations doesn't work with structured alt keys. The only exeception here is a structured alt key with only one component.
For you scenario I see two options:
Option 1: In addition to person_otype and person_objid add a new field person_key with type /bobf/con_key and configure the association using this new attribute. Then you write a determination which fills the person_key. You can determine the key with the CONVERT_ALTERN_KEY method of the service manager for your person BO.
It doesn't make a difference if you add the person_key as a persistent or transient field. Both variants work fine. Only the determination configuration would be slightly different.
But because you mentioned, that your person BO has an own dac where you generate transient keys, you should add the person_key as a transient field and configure the determination as transient and use after loading and after modify as evaluation timepoints.
Option 2: if you don't want to add the person_key to your structure, an alternative would be an own association class. There you would also use the same logic (convert_altern_key) to retrieve the key for the person.
I recommend option one. I did this variant a lot myself, also with an custom dac which generates transient keys.
Kind regards
Aiko