The diagrams on modretro seem to be gone, so I figure I should post this up here for my own reference. The DPDT wiring to have a power jack to charge and a power jack to play is as follows

 

This results in: Center off, top to play off of external power, bottom to play off of battery.

 

This will allow you to view battery levels

 

This could be used if you want only one charge port. But would require a powerful smart charger.