How to measure small current waveforms using a uCurrent Gold and a mains grounded oscilloscope
I also have a uCurrent Gold and it is super handy. 

I also have run into the problem of wanting to measure the current between two points, neither of which is ground. 

Since your circuit under test is battery powered, you could just connect the ground of your scope and the uCurrent to P2 - if you are willing to drop the voltage measurement at P1. The current measurement will be right.

When I really want to see the voltage and current at the same time, I use two channels on my 4 channel scope with one at P1 and one at P3 and then I difference them to generate a math trace showing the voltage at P1 relative to ground on the battery powered circuit. I then put the uCurrent output voltage on one of the other remaining channels.