While your idea to spawn some random patrols outside of the designated AO area is a sound one, the over-all idea will be limited by a few known issues.
First, AI units do not innately understand what a building is and how to occupy it, use corners for cover, check doors, etc. Even if they were to spawn inside one, chances are they would disregard its tactical value and enter search-and-destroy mode, spreading out into the local area. Either that, or they would have a pathing error and simply stand still with their barrel pointed into a corner, making it very easy to sweep the building and eliminate them.
Second, I&A on the whole uses a spawning system for both enemy units and placed stuctures/objectives (side missions) that cannot and likely never will understand the difference between a building and any other location of terrain. To the best of my understanding, it functions like so:
With this in mind, while it would be possible to spawn random squads around the map, it would be next to impossible to have them spawn inside random buildings between you and the AO, that would be like trying to take a blind-folded swan dive into your shot glass.
The best that could be done, in theory, would be to designate specific coordinates to tag along side each individual AO that spawn AI units in specific buildings near each AO. Problem with that is that each AO would have its own list of buildings to pick, they would all have to be EXACT locations, and they would be the same each time you ran that AO, becoming extremely predictable.
