Springboarding off of this idea, has anyone ever played Tropico? It's a game where you're a dictator/el presidente of the carribbean island Tropico. It has a similar population system to DF in that you gain people by either having them be born OR by attracting immagrants based on your islands prosperity. I think it has a good handle on immigration/emmigration, so here are the basic points.
#1 At the start, immigration and emmigration are uncontrollable. You have no control over who comes and who goes. This is similar to DF now (except there is no emmigration system). Basically unhappy workers either choose to stay or leave depending on their personality traits. Hostile and stubborn people stay while passive and compromising people will choose to leave rather then "stick it out".
#2 WIth the building of a structure in Tropico (or in this case a noble perhaps, perhaps your own Liason to travel abroad) you get the ability to control immigration and emmigration to various degrees depending on the skills of the worker assigned to the immigration office. You can choose to
A) Increase immigration overall, with the potential of add +50% to immigrant waves with fully staffed, max skill immigration officers
B) Increase skilled workers. In DF this could mean selecting to focus on a broad category (Miner, Woodworker, Stoneworker, Ranger, Metalsmith, Jeweler, Craftsdwarf, Administrator, Fishery Worker, Farmer, Engineer, Military) and have an increased chance of attracting members of that profession (in Tropico, maxed out skills and workers resulted in +25% chance of having a "Novice" and a +15% chance of having a "Specialist" (i.e. the people who just have one skill at name value)).
C) Increase emmigration. This resulted in a greater percentage (+25%) of unhappy workers had a chance of leaving rather then going beserk (in Tropico they become rebels and fight against you instead of just going berserk and killing everyone...actually it was pretty much the same
)
D) Stop Emmigration completely. With maxed out skills this would block 100% of emmigration, meaning that nobody would leave.
E) Stop Immigration completely. With maxed out skills it would block 100% of immigration.
Now the challenge was you could only select (1), so you had to decide what you wanted to do. I think this logical system could easily be applied to the DF model (actual code is a whole 'nother matter) in that the system worked very well and allowed different people to customize their game experiences. This could be a setting off of the noble screen where you appoint an "immigration administrator" who can choose to work on one of the above options. It would use the broker skills, although the gaining of it would be based on successful use instead of just flat skill points like the record keeper is. I.e. whenever the immigration officer's influence actually changes the outcome of an immigration/emmigration wave, the immigration officer would gain XP. Similar to how marksmen gain XP when they hit stuff.
Once overland movement gets established, you could seperate the immigration and emmigration officer duties. The emmigrator officer could just be the mayor, who depending on his skill urges dwarves to leave rather then go beserk and kill everyone. He'll do his best to make the angry dwarf happy, but as a "last resort" might be able to mitigate the angry deserves beserker tendancies. Meanwhile, the Immigration dwarf (now a Liason) would actually travel the world map to the other dwarf cities (and maybe elven and human once that gets established as well). You can assign him guards and they'll leave for X seasons (2 makes sense), go out and travel and influence and possibly get eaten by skeletal giant eagles, but if they make it back they'll bring back either
A) An extra batch of immigrants, same old same old except you might be able to get 2 batchs in one season this way
B) A few highly skilled immigrants (one or two ranks up from your typical immigrant)
C) Non-dwarven immigrants (maybe a human woodsman or an elven archer)