I have no idea, still working on this, but you can always use a wireless keyboard like the rii mini i8. It will set you back about 15$ but it will work for sure and doesn't need holes or the like. It only uses 1 usb slot
I have tried my best to make it so that you can handle everything inside piplay without a keyboard. you can press 'tab'/joystick button 3 in the piplay main menu to change the volume and access other functions. That being said, I totally endorse the hausbell rii mini i8. it works as a trackpad and keyboard, charges fast, has a long battery life and auto sleep. I own 2 of them.
esc, enter, and menu are pretty much my main ones. I set my arcade so that player 2 buttons are mapped to 'e','x','i', and 't' so that if something goes wrong, my son can get back into piplay from the command line without rebooting or a keyboard.
My arcade cabinet is about 99% done and I still use a keyboard for a few things. Sometimes it's just easier than trying to remember what button does what on a joystick console. Either use the mini keyboard or do like I did and set up a hidden USB socket somewhere easy to get to. The plus to the hidden USB is I can charge my phone on it too.
I had a thought today on the mame title selection. I love the player button to select by letter. I think it could be made quicker if once you press the button, left and right on the stick moved through the letters like it does now, but either up or down on the stick selects the letter. No need for a second button press.