To qualify this: I am not sure quite how you would do this, only that I am pretty sure it can be done (depending on your version of Notes).
I would suggest not trying to give anyone access to make modifications directly in your Directory. This is the definition of “unintended consequences,” as I can’t think of any way you could allow a user the ability to modify only one page in a form in a database. That’s not the way Domino Databases work.
However, you could create a form that would trigger an agent to update the information you want them to be able to update. The receptionists put the information into the form; the agent operates behind the scenes, much like Admin-P works to update a user after a name or OU change. They can make the modifications you want them to make without having, themselves, the ability to monkey with anything else.
Does this make sense?