So I went for a test ride. Here's what happened:
Looking left for incoming traffic I hit the mount and the phone dropped face down on the tarmac till I picked it up. Luckily undamaged (the mount kept it scratch free). So I cancelled the project and made a new DIY mount for the bike. Here it is:
Better than the previous one I had, offers space for more gadgets or other stuff if needed and the slightly elastic metal I used promises some shock/vibration absorption. I'll test it on first chance and hope it really does so.