Just had an idea, maybe not the wrist for mobile devices, maybe a ring that is always on the hand that is on the back of the phone. I don't know, but I can't help but think of how things can be more secure and that there is a market for those with security in mind.
Just found this also in my search while typing this comment.
I don't like rings (you put on your fingers). But that's just a personal preference. I'd be okay with wristbands though. Yes, the proximity to smartphone NFC readers would be a benefit of a ring.
Just found this also in my search while typing this comment.
http://nfcring.com/
Looks like it might be open source as well?
https://github.com/mclear/NFC_Ring_Control
Might be something to keep check on, it supposedly doesn't release until mid 2017.
EDIT: Just thought about if this is open source, anyone could possibly tie it in with automation apps such as Tasker and really do neat stuff.