My immediate answer is "GlassFrog" - Lead Link would capture the name of the person he has assigned to such role.
Secondly, there is nothing in the constitution re the fact that the Lead Link should make explicit how he selects partners to fill roles. This role has got the authority by an accountability (see appendix A of constitution) to do so.
In case, tension surface re the way a specific Lead Link assigns partners to roles, then process it. There are so many different ways to "solve" any tension.
For instance, within iGi Partners, the company I work with and using Holacracy since 2010, there is nothing explicit per governance re the "How" should Lead Links of our 5 circles assign roles.