Has there been any update yet?
I think a neat compromise would be to get rid of the volume redundancy and e.g. only have volume up on menu + dpad right. Personally find left/right the most intuitive because it also mimicks the volume UI in the side menu. That would free up the other two dpad buttons to be assignable via the SDK, or they could trigger an event similar to the pause or resume events.
I think this would be a clean alternative to button combinations or input schemes via the regular buttons, especially if a lot of those are already used for other ingame actions. Examples for common custom actions are opening up map or inventory screens which would naturally fit the menu button anyway. A risk could be that developers may rely on that additional "easily accessible input" to make their games' controls more complex instead of trying to design around fewer buttons which might not align with the Playdate's intended design direction.