If your crazy enough to do that.
And if your crazy enough to have gotten others crazy enough to do it as well .
Note, under all circumstances, these trials were done with the batteries fully charged, and once off the overnight charging, voltages drop more.
Chris, with his Mean Green, sent it back as once off the charger he was dropping into the 9's for starting voltage, but may be influenced by his batteries although they were tested to be "OK".
I'm definitely not and I hope others aren't either. Realistically it's unnecessary wear and tear on multiple components. BUT more importantly, I'd argue to do any measurements under real world conditions. Not w/ daily fully charged batteries but after a truck has sat for a week or so unplugged and it's 10* at night and only 40 during the day. No battery charger no block heater. If I drive to work, the store or anywhere else the truck is unplugged.
I could do a measure of battery volts and whatever else I can conjure up. Then I think we can take into account, IF there is a significant negative affect, it would be more noticeable under those conditions. Your measurements say there is a larger drop in voltage but I wonder how it will affect the truck. With healthy batteries and alternator i'd say at the moment, it's worth the risk.
Fwiw, even w/ 13.6v and on a battery charger sometimes my old starter was sluggish. I had more than 1 time in the winter, unplugged that It took 2 cranks to start. The glow plugs and wires are all new, less than 10k so I'm confident on those.
How many amps will each bank on the glow plugs use? I should check that as well.
AND, you do electronics well, that being the case.... since the alternators on these trucks seem to take several secs to ignite and only to 13ish volts, how would you change out to an instant on voltage regulator at 14+ volts? The parts are out there but will they fit on our factory alts?
Would something like this work?? https://store.alternatorparts.com/partnof600.aspx
No soft start, so it should be instant ignition like it is on my suburban and durango? Straight to 14.5 volts would be the goal.