you'll likely not get the effect you want.
Sure enabling environment maps on a mirror is a good idea, however a true mirror reflection is not how they work. you'll never see a reflection of your character in the mirror for example, they are fairly limited in some sense, the reflection maps are normal cube maps/ sky boxes. They cannot be swapped out dynamically without major scripting and painstakingly making one on a per-location basis.
the idea would be, make a detailed yet featureless env map, that way you won't have a cactus refecting on your helmet for example when you are inside a house. you can mask them off a little- subduing the images reflection ie you can't quite make out the cactus, but still giving that surface reflection.
There is still a need for someone to create a more detailed reflection maps based on the generic feature less vanilla game ones. also I want to do a generic map but that is brighter on the upper part- which will likely be a good match for 90% of the game, as lighting is almost always coming from above the object or character
be aware they a rendered in object space. so if you do a fairly detailed one, or want it to be lighter at the top, you have to take that into account. ie for some weapons which are rotated 90.... so you might want to take that offset into account if you make a specific one only for use on guns. and make the map to compensate
I released those 2 maps as a kind of poc for F3. Up until about nov-dec last year, that I am aware of, no one knew or at least had created custom reflection maps, and were dumping normal textures or reusing the few that shipped in the vanilla game. So I set about rectifying that. writing a tutorial and releasing a couple maps.