I tried to do what you were thinking (point the remote display up at the window so you could see the reflection).
I found there was no great surface to mount it to to get the angle it needed to be at to reflect properly, no clean way of routing the cable (that wouldn't have involved disassembling the dash and HUD) and that with my height/eye level I had to position the remote display above the hud, which actually put it at a kind of obnoxious height while driving.
I could have put it further over but the NY registration stickers were in the way so I just stuck with the steering column. I like being able to hit the mute button without leaning over and touching the V1 itself (because usually the suction cups give way and the thing falls off).
2013 335i M Sport /// Alpine White/Coral Red