Having laid hands on a second PiZero [1] I can now commit it to the final build steps,

• An On/Off switch
• Shorten the wire runs and make thing more compact
• Optional, Add a switch that can be coded for on the Pi. Uses for this could be ...
  

• to do a proper sync sync shutdown dance before the powerdown
• to do a full data wipe/encryption should the need arise
• skip a track in the playlist
• jump to a secondary fm freq
• etc etc

[1] Yes, PiZeros exist. No they are not being horded by various distribution points in order to help the NSA build up the panopticon or to help set up the fake Mars mission that will rival the old Moon Landing Hoax. Please, please please, stop harshing on resellers and distributors . Get on a wait list or lists, jump when you see them go in stock.