First and foremost: dwarves who are soapmakers by trade that immigrate to your fortress only to get stuck hauling corpses out of your deathtraps and hacking out tunnels should not be happy. I think it would warrant an unhappy thought even. Furthermore, these types of immigrants are useless to the player and only the most desperate dwarves should immigrate to a fort where their trade skills aren't needed. Dwarves should have some idea of what kind of work your fort needs done and seize these opportunities when possible so they don't get stuck scraping goblins off the underside of your bridges. More on that in a bit.
How would this work? Well, one of the game's quirks that makes this difficult is that forts are somewhat isolationist, so it's hard to spread word about anything. The only people that come to the fort are immigrants and merchants representing entire civilizations. Two things would help to make the fort seem a little less isolated: visitors and independent merchants. Visitors would mostly be friends or relatives of your working dwarves and would be more or less inclined to come to your fort depending on the distance to your fort and how dangerous the journey (and their stay) would be. They mostly wouldn't do much and there would never be many of them in the fort at a time (possible init option?). Before the economy has kicked in, they could meet with an immigration officer and pay your fort in coins or anything else of value (perhaps they could also work as peasantry for part of their stay?) to pay for expenses like food, booze, and hospitality. They could be ordered to stay within a certain burrow (in the next version of course) and would explore wherever they're allowed to go and perhaps gawk at a few engravings. If they have any friends or relatives in the fort, they could meet with them in the meeting hall when they're not working, and if the dwarf is on break they could chat it up in the dwarf's quarters. Visitors could give your dwarves happy thoughts, or unhappy ones if they don't have enough time to talk with them. They might stay for a month or so.
Another type of visitor would be the independent merchant. These merchants would not represent a civilization but would work independently. They would not have a liason and they wouldn't carry as much as the civ merchants, but they would be more numerous. There could be any number of them at your trade depot and you could meet with them at any time. These people probably wouldn't do much more than ordinary traders would. Both types of visitors would probably be spreading word of mouth and delivering messages (that's what they did back then, isn't it?).
...But what this is all leading up to is the immigration officer. The immigration officer would be a noble and would allow you to control whether your fort needs immigrants or allows visitors, and would also allow you to control what kinds of immigrants you get. Basically, he would welcome any visitors by conducting meetings with them, providing any information about their stay and answering any questions they might have, acting as a sort of information vendor. He would deal with all the fort's immigration and tourist issues. At any time on the noble's settings screen you could adjust how badly different skills are needed, similar to how you can adjust how much of a particular good you want the caravans to bring. Through word of mouth surrounding settlements will have an idea of what kinds of job opportunities there are at your fort and immigrate based on this. There might be shortages of available nearby workers for a job you need to fill in and peasantry or dwarves with unneeded skills could still come - you don't have absolute control over it all, but you can get word out what kind of workers are needed. I think this would add to the game and might be a little more realistic as well, people go wherever there are opportunities.
Thoughts/comments?