Not to say that this doesn't make sense, but on an N Android device, holding the volume-down button appears to keep the screen on.

If we want to do this, it'd possibly not be too difficult to make Chrome avoid reporting autorepeated volume key events as user activity to powerd. On the other hand, the power button is right next to the volume buttons, and it's impossible for us to ignore the volume button being held down on current hardware (since it does a hard shutdown after eight seconds).

This problem is more generic to all tablet form factors (convertibles, detachables, slates). 
Before we start thinking of how to react to someone putting the device at an angle and forgetting about it, I'd love to see if this actually ever happens. Do we have any stats to show how long people press the volume buttons?
I wouldn't be surprised if people notice that device didn't shut off and just straighten it.

I'm not aware of any stats related to how long the volume buttons are held.

I could believe this is a rare enough problem that we don't want to solve it; I just had it happen to me yesterday and thought it was worth making a decision around.