I see.
This is not possible with Cook.
To reduce the description simplier : (You do not need to involve AJAX groups to raise the problem)
Make a simple FK in a form wich to refers to an account, itself referring to a Joomla user (double FK)
The result shown in the form will be the ID of the account, because cook does not accept more than one level deep for the labels in form combos.
This is interresting because I never had this case, and it can be usefull sometimes. But it becomes really complex to code. Good challenge.
It would be a new functionality, but I put it in pending tickets, because Cook is not supposed to limit deep levels when context allows it.
When you drop a sub/sub/sub... node in a form, it already reduce it to the base (which is normal from the context), but it should keep the choosen node for its label.
Right ?