Achievements would be trivial(-ish) to set up through lua. You need various icons, a text file containing the various achiev's, another file containing the data for the achievs and voila.
All this talk about the achievement, it seems like none of you have earned a single campaign medal...

FS already does have an achievement system in place, you can get badges for kills, completing mission objectives... Pretty much achievements, only better integrated into the setting. Steam could track of those (like it does with Tropico 4, for example) in it's own achievement system, but that's it.