That is a good idea and would be easy to add. Could I ask what key you would feel makes sense for "wait"? I am thinking maybe the . key?
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I would probably use 'spacebar' myself. But that means it's easier to whack the thing on accident, of course. (I use spacebar because I often play melee and want to wait for melee monsters to walk into my range.)
In NetHack, "rest on spacebar" is an optional setting, with period as default. So the period key works well!
Since s and x are both down, you could use one of those. I would say s makes most sense, then you've got the 8 directions plus the middle button, but people are so used to using wasd that it might be easier to get them to wait on x
You could also get people to use the numpad for movement, which would make it pretty intuitive. Not everyone has a numpad though