Pat....
I wrote this up at one time and it is probably still posted around here somewhere.
It really was an accident in how it came about. I was cooking for a group of buddies and when the butt was finished, we weren't ready to eat. So, I foiled it and threw it back into the smoker. I then closed the cooker and let it sit for another several hours...8 maybe. Yeah, we ended up eating this thing by early morning moonlight! LOL
Anyway, it was among the best butts I had ever had. It had NO fat whatsoever and was still very moist. I didn't measure yields or anything like that, so not sure of that impact.
As far as when I foiled it...I have not used a thermo to measure meat temps in a long time, so I am not sure what the temp was. It was very tender...use the fork test and twist. It had been cooking for my standard 2 hr./lb. and it was an 8 lber.
No idea if the fat continues to render after it has reached 190-200? and then cooled back down. I can tell you that this one had literally NO fat when we cracked it open. This evidence seems to support your theory.....the fat continues to render after hitting peak temps and cooling back down very slowly.
I cooked many more butts that summer in this same manner and I am convinced it works pretty darn good. Plus, I am not very good at math, but planning something for 24 hours is easy even for me!