Have some fat wrist porn: https://www.dropbox.com/sh/kjed7wq0p23l42h/RbCpiuaGjH
I don't usually work directly on my MBP, being of the one true religion (external keyboard/mouse/monitor), but when I do, the first is approximately how my left wrist normally rests, and the second is with my Pebble on, showing the buckle in contact with the MBP.
It was hard to get the angle right on the second picture, so the position of my wrist is slightly off, but you get the idea.
When actually typing (especially for more than a couple words, like I am now), my wrist isn't usually resting so heavily on the surface, but the buckle is still in contact with the laptop.
The scratching is annoying, as is the pressure on my wrist. This has been the case for every watch I've owned with several different straps, so I almost always take them off when using any laptop without an external keyboard.
Personally, I find my watch tends to sit above my wrist, so it's not over the palm rest at all.
In other words: This is how reality works. And here in reality, the buckle is annoying. Speculation as to its annoyance in an alternate reality is beside the point.
I've had carpal and nerve issues in the past, so I watch this a lot... generally you can drive the better habits simply by reconfiguring the work setup.
But all of this is quite irrelevant when one is talking about the inherently non-ergonomic use of a laptop that only happens in meetings and hotel rooms.
http://mindfulmotionphysicaltherapy.com/what-is-an-ergonomic... (For info purposes - not endorsing this particular practitioner.)