just curious if there's a mod that adds mirrors, cause I haven't seen one yet

True mirrors have two major problems. Phenomenally hard on the video renderer, because it has to render everything twice, basically, and you have the problem that the player's body doesn't actually exist in First Person view.

so it's possible, however very difficult as you need to adjust a model for the first person view, I've heard of a mod that did just that, made it so you'll see in third person and the camera was zoomed in so you'll see as though you are in first person when you're actually in third person view.
As for the issue of rendering twice, major frame-rate problem, probably lots of lags, maybe it's possible making the reflection's quality lower than the quality it renders everything around it?

Possible, probably. Like you say, you'd need to render the player first, then make sure that the player's body doesn't interfere with the camera (or only render players body in the "mirror" image side). Lower quality probably wouldn't be the way to go because if they were different than the ones in the real world, the game would have to load two different ones instead of rendering the same one twice. Though that itself may not be possible because the game would really have to render mirror image versions of all the items in the real world, so it's probably already a matter of loading things twice. Even Portal didn't have to deal with that.

What is needed is true reflective surfaces, and I don't think the game supports them. Maybe OBGE could pull it off, but I doubt they'd be useful in practice.

First of all you'd have to find a way to actually make everything being rendered twice. The only part of the game that has reflections is water - and I think utilizing water is the only way to get a mirror. You could make a water type without any ripples, wind etc, 100% opacity and 100% reflection. That should look like a mirror. Then you'd need to adjust the ini to make the game reflect everything and not just light sources, landscape and _far nifs. If you used the 'lava' material you could make the mirror a mesh as well, but I heard the material stops working if the mesh it is projected on is not a flat horizontal plane. Maybe worth a try.

The water plane works perfectly well vertical, I've got two mods that do just that.

Using one as a mirror sounds like an interesting experiment. However it is worth noting that the water plane mesh only works if a cell has a water type assigned. So if you planned to have 100% reflective mirrors in a cell somewhere, you wouldn't be able to also have usable water in it unless it being 100% reflective wasn't an issue.

Given that - I assume it would be safe to say - hardware tends to be pushed harder outside than in, shouldn't this be quite viable?

I mean, if it even works at all.

I would like somebody to really give this a try. Would be pretty interesting.

Definitely would not want this mirror outside though. In a small building it might be possible.

my thoughts exactly, it's an interesting experiment, and if it works I'm sure it can be used in tons of mods.
and yeah, of course not outside.