Actually, you can, for example Amazon has this:
https://www.amazon.com/Magnetic-Charger-Neotrix-Wireless-Standard/dp/B01GYPMECG
Of course, it won't work well as you can see from user reviews, for an obvious reason: you can't cover the wireless charging coil by steel disk, instead you must rely on a pretty thin steel ring which doesn't cover the charging coil, meaning it won't have enough force to hold the phone well, especially on a bumpy roads ;-)
These vent mount magnetic wireless chargers work well. Put it on the bottom of the vent right of the steering wheel and they don’t move or shake while driving. Plenty of strength to hold the phone too. Convenient to see what happens on your phone without taking your hands off the wheel.
For hiding the cable you can use your key to wedge the wire between the center console and lower gauge cluster / steering wheel trim to the floor then run it under the center console trim to your power source. No cables and a clean look.
Ran this in my Mustang beautifully. Lots of people asking about it. Went to a standard magnetic vent mount and plug in lightning cable on the truck so I can use CarPlay.
I have this magnetic wireless car charger for my S7. It works and doesn't interfere with Samsung Pay or my other wireless chargers. The only problem is that it charges pretty slowly. It's fine if I'm not using the phone for anything while it's charging, or using it just for music, but if I'm using Google Maps or Waze for nav then the phone uses more power than the charger can supply so the battery still drains.
Edit: in case anyone just skims this post, the link below is to something that doesn't work that I tried.
I'm wondering that too. I switched to iphone x from a Lumia 950xl yesterday, and I tried it with what I had been using before (https://www.amazon.com/gp/product/B01GYPMECG/ref=oh_aui_search_detailpage?ie=UTF8&psc=1) but unlike the lumia (which seems to weigh about the same), the iphone doesn't stick. It slides right off (the lumia never fell off, even on bumpy roads and fast turns). The magnetic pull between the iPhone and the plate is much weaker for some reason. Maybe the iPhone's qi coil is smaller or further from the back surface of the phone? Not sure if there's a limit to how strong the magnet can be but I think a car mount would need a much stronger magnet for this phone.
I bought this one and it works perfectly. Instead of a plate it uses a ring. The ring allows for wireless charging both in the car and on the pad at home.
Magnetic QI Wireless Car Charger Mount, Neotrix Mobile Cell Phone Air Vent Magnet Car Cradle Charging Holder for Samsung Galaxy Note 8 S8 Plus S7 S6 E https://www.amazon.com/dp/B01GYPMECG/ref=cm_sw_r_cp_apap_uJmcE1DmQtA9O
I think there are a few. This is one I found just by chance.
The main issue with magnetic holders is that wireless charging doesn't work well through metal (hence the glass backs on the 8/8+/X). This mount seems to use a ring of metal so charging can still be done through the middle but it'll require more precision placing over the regular magnet mounts that just use a plate.
This one is a magnetic mount with wireless QI charging. It includes a flat, metal ring you put between your phone and case. The ring has a wide enough hole to still allow for wireless charging
Was super confused if I'd gone back in time, this was posted yesterday:
That's a fairly expensive case that can do the trick, or you can use this: https://www.amazon.co.uk/Magnetic-Wireless-Charger-Neotrix-Standard/dp/B01GYPMECG/ref=sr_1_1?ie=UTF8&qid=1509464007&sr=8-1&keywords=wireless+magnetic+car+charger
I can confirm that it can hold the Note8 without a case even over bumpy terrain.
I bought this which is similar, but also wireless charges the phone, it's brilliant!
I am currently using this wireless magnetic charger.
It comes with a thin ring that you can put in your case, the hold is strong and you can still wirelessly charge, obviously.
It isn't a fast charger, but in a 20 minute drive I get about +3% battery with bluetooth on, music playing, navigation on and brightness at about 80%.
My dash isn't conducive to suction cup holders and my vents are weirdly shaped so that doesn't work either. I had a couple cheap CD slot mounts that I use with it instead and I love it.