After confirming with Google, we've learned that Android Pay for Android Wear will not function if your paired smartphone has an unlocked bootloader. This is not surprising, but it is disappointing nonetheless. This is despite the fact that we know the actual Wear app runs totally independently on the smartwatch, with the phone at most acting as a data pipe over Bluetooth. But the phone is integral in the actual setup process for Pay, and we suspect this may be where the limitation comes into play.
If you have an unlocked bootloader, the Android Pay app on your phone won't even let you add a card, and setup will fail, notifying you that your device is unsecured. Which is to say, this is 100% intentional behavior. Even if your connected Wear device does have a locked bootloader, the phone's bootloader status apparently is enough to be an issue.
We've asked Google why this is and if this limitation will be permanent. The answer we received during our time with Google checking out the new LG Watch Style and Sport was not encouraging in this regard. There was a lot of vague discussion about how the payment networks are very strict about this sort of thing, and that it's out of Google's control, but we didn't get anything in the way of specifics or, more importantly, any indication that Google was interested in addressing this.
Re-locking your bootloader will, of course, fix this. Granted, that means wiping your phone, which is exactly why so many people have complained about this problem in the first place. Long story short, if you want Android Pay on your phone or otherwise, you should lock your bootloader. We've asked Google for a more detailed explanation, so hopefully we'll at least have some of the reasoning to give you soon enough.