I don't read the suggestions forums very often so I don't know if this sort of idea has been proposed before. Basically, I want there to be some sort of in-game control over immigration along the same lines as the trade contracts with the mountain home.
Have the liaison meet with your Outpost Manager and start up a request screen as to how many, and what kind of dwarves you would like to see in the next season. Maybe have it on a point system, or a priority system, where you might want to get a handful of craftsmen, or a large number of peasants or some such. If finding enough time to meet with the liaison is a problem, maybe split it into two separate visiting nobles, a liaison and a trade minister or something.
Along the same lines, I'm a bit annoyed that the liaison meets with the Expedition Leader instead of your broker for the trade contracts. This is mostly only a problem because my leader title was assigned to a random miner instead of my trader, but it still makes more sense to me that the broker would have more to do with trade contracts.
To help with the annoyance of having the liaison follow your dwarf around waiting for a chance to butt in, maybe have them auto assigned to travel to the relevant noble's office and wait there, like traders do. Then, like the depot, give the task "noble requested" to priority drag someone into the office to interact.