Wouldn't it be possible to display it in the mirror in the same way as Bve5 in the future?

That's quite an interesting effect.
It can't be a 'true' reflection, as BVE5 doesn't actually support an external view.

Similar (I would suspect the same basic technique) can be achieved today using an animated file, a set of appropriate textures and the trackdistance function.
Basically, you want to combine the textures into 2 or 3 filmstrips, and the TrackDistance function to switch between them. (As you'll want a greater difference between frames at distance)
The TextureShift function can then be use to move the appropriate frame on the filmstrip into view.

Could also do it with lots of states, but the TextureShift approach would likely be better.

Someone has already done this; I can't immediately find which, but it's probably one of the HK routes.

I asked him directly about this, but he draws the 3D space frame by frame only in the mirror area, intersepting the Bve5 drawing and drawing directly.
As you know, Bve5 is closed source, but it is made possible by building a system that can be accessed from the outside.
OpenBVE does not require such effort as it can be implemented directly as a main body function without going to the trouble of doing so.