While my mind was going through all sorts of things for guard rotation and the like, but then I realized what was really needed was a building.
Basically you build a sentry post where you want to station a guard, 1x1 would probably be best, but it could be bigger if that would work. Military dwarves could be set to sentry as a job (or maybe opened up to all dwarves as a possibility, but should be open to military) and they would arm themselves and go to the post and stand sentry. It takes a set amount of time to complete the job and you can set it to recur. It probably doesn't increase any skills and it definitely doesn't get completed faster by more skilled dwarves. Since I'm not sure how the job task works, if one dwarf leaves in the middle of the job, another dwarf should be able to take his place.
Another way to do it would be to make standing sentry something that members of a unit would do while standing down, like weapon practice, and the sentry post is the object they interact with. This doesn't make as much sense, but it might potentially be easier to implement and avoid having active soldiers leaving their patrol to go stand sentry.
I believe that currently you can station soldiers at a point to have their unit go there, but then I think you have to deal with the rotation yourselves, whereas with the job mechanism you would get 1)If there are more sentries than posts, those without a place to stand sentry would take care of other tasks or needs, 2)Sentries that need to take care of needs could do so and be replaced by others, 3) Sentries would go to the nearest open post, rather than a specifically assigned one, though you can make a spot more likely to get filled by putting a barracks nearby.