As long as the magnet isn't towards the bottom (covering the wireless coils) it should be fine.
Magnets cause the S-Pen to not be accurate, though if it's just a piece of metal it won't change anything IRT the S-Pen unless docked.
I always liked these (Nite Ize Original Steelie Dash Mount Kit - Magnetic Car Dash Mount for Smartphones) but the magnet causes the S-Pen to lose accuracy when around the magnet.
This magnetic one is what my partner and I both use. It's worked really well for both of us.
I have had this mount for my iphone in my truck for about 2 years and have been very happy with it. I am ordering this mount by the same manufacturer to attach to the H-rail on my hobie outback.