That's the right idea!

It struck me that if you could compare all the cylinder mV readings quickly while driving under different conditions, then that would be useful!
You can utilize the thermocouple inputs on a cheap Harbor Freight multimeter to get a very responsive 3 figure reading from the glowplugs for comparing to the other cylinders (it doesn't matter whether it is a true reading here...) Here is what I was planning on doing, but someone else could beat me to it:

I bought a cheap HF multimeter with a thermocouple input, a spare glowplug multi pin socket off the drivers side fender mount at U-Pull, and a 5-way rotary switch that I got from the local surplus store.
I planned to wire the glowplug socket via a multi-lead cable to the rotary switch - with labelled detents according to which cyl is being measured, then wire the center tap of the rotary switch to the thermocouple input on the HF meter.

This way you can start your car normally, and warm it up - then while it is running, unplug the glowplug multi connector from the controller, and plug it into your modified harness which is routed into the passenger's compartment where you have an assistant with the meter and the rotary switch. Then go drive the car and compare the cylinder readings under different loads (with a steady foot on the throttle)

I know what you are going to say - too much work!!