I just thought of an ingenious way to allow for mirrors in all editions, even standard. It does have it's limitations though, but it's much better than a mere screenshot used as a texture and much more accurate. It still won't, however, be like real mirrors, but very considerable. This only works with flat mirrors and not a room filled with them (thus, for a typical bathroom mirror, this would work). Here's the trick:

1. Where a mirrored block would normally be, have an opening that goes into the wall.
2. Recreate the scene that would normally show up in the mirror having depth. Only, scale it down, way down. The smaller the mirror, the smaller the scale should be. Use lower-poly models than those in the scene. Make this replica passable so collision isn't run on it to speed up the engine.
3. For the player, just have a duplicate in this area on the "mirror" and, of course, scaled down accordingly.
4. To prevent weird things from happening, put an invisible barrier right where the mirror would normally be to prevent the player from getting in it. To avoid seeing the "special effect" when in a remote area, make the mirror scene invisible and passable.

Does this work for you? It should work even in standard edition even in very old releases like A5 or A4.

Edit: I haven't tested the trick in Gamestudio, but from running simulations in my mind game, it should certainly work.