I mentioned this in another thread myself (“More Brothers”), and it’s definitely my favorite solution to the problem of having horribly wounded brothers sitting in your army, or brothers that are poorly trained to deal with the sort of battle you’re running into. I think inns would be the best place, or a mercenary guild of some sort, but not a place that you own. It should definitely be a place that you owe rent, and you should only be able to have so many brothers on leave.
Maintaining reserves is definitely something we should be able to do in Battle Brothers, though I don’t think those reserves should be very largely (I lean toward the number 6 as the size of your reserves).