The last bits of soldering I needed to do today were:

• Hook up power to the pi pico
• Solder on a couple resistors for the voltage divider to the ADC pin
• Solder the enable wires for the lipo shim and the master power switch to the pico.
• Solder the uart wires between the pi 3 and the pi pico

This mostly workd great.  I was able to use the pico serial console to switch the pi 3 on and off in software and measure the battery voltage.  Also I re-tested all of the buttons and stuff to make sure I didn't break any connections. 

However, I had screen flickering when the pi 3 was on... turns out that something to do with the uart pins having wires on them is causing a prob.  I'm not sure what yet...   I tried with the dsi display and hdmi and same prob on both.