I like this suggestion, were it handled properly. There're a few suggestions to add to this:
1). The Liaison's visits could include a "hire" option, so you can pay (in trade goods--I don't like the idea of coins) for the services of a dwarf. He could have a contract with the Fortress for a default set of years, with the contract renegotiable every so few years. You could view their equipment, skills, and possibly gleam some information from them through Legends, and determine it that way. This way, you could hire out unusual mercenaries or request certain jobs filled. There would have to be an option to "buy out" their contract, so they can stay indefinitely. Human liaisons might also be able to do this. Though a tad complicated, and in need of some work, I'd like to see this feature.
2). If dwarves leave by their own accord, they should go into a meeting with the leader; a certain list of requirements, certain mood tags, and a lot of random chance should determine if they want to leave by their own accord (so you won't potentially lose your whole fortress, though you might.) The dwarf, if he wants to leave the fort, might enter a meeting with the fortress leader (whatever rank he is); the fortress leader might persuade the dwarf to stay (at your own discretion--the dwarf might make a demand to want to stay, so you'd have three options: Reconcile based on the leader's social skills, meet the demands such as they demand stately quarters or their own private dining room, or simply let them leave.) Optionally, your leader might put out a contract for one of your dwarves to the liaison, so you can send some away for a while.
While this is fathomably more complex, 'twould be pretty damn cool to see.