The game data contains leveled list outfits for five of the game's seven hireling followers. Ordinarily these are not used and are replaced with a fixed outfit. However, the unofficial patch restores them. The following are some observations on how these lists are used after patching:
Data pages:
One might expect this outfit to be determined and fixed when the NPC is first loaded, but the reality is more complicated. From my empirical observations, it appears that their outfit does not become fixed until they are hired for the first time. At that point the game adds their inventory to your save file. Before then, the outfit is rerolled each time the game is loaded. The determined outfit is kept in memory and can't be changed by simply reloading a save, the game must be closed.
Thus if playing with this patch, if you want to try to choose their default armor, it would appear to be sufficient to simply not hire them until they appear with the chosen outfit. There should be no need to avoid the area where they spawn.