Is there a way, similar to denoting keyboard shortcuts, to denote buttons on SE posts? I thought it would make things much more readable.
No, there isn't.
If you don't want to abuse button or button or button or button or button or button you're out of luck.
The very creative could probably abuse some unicode but I guess that is about it.
<kbd>button</kbd> [tag:button] **button** *button* <sup>button</sup> <sub>button</sub>
Or pick some other options from the allowed html to see if you can construct something that resembles a button better then my futile attempt.