Quite a while back a very similar "sunburn" happened to my viewfinder, only this was in the Galapagos, and it was a Sony PD-150 in an Amphibico housing. Expensive fix.
I thought at the time it was during a short ride when the housing was on the floor of the Zodiac, going to a dive site. The housing had a sort of magnifier on the viewfinder which I still think made the burn happen much quicker.
After the repair I searched out and put a small rubber cup over the eyepiece whenever the thing was topside. Never repeated the problem.
Amphibico, alas, had "never heard of the problem" and didn't have any cover available for the viewfinder.
I think it is a good precaution to take to cover the eyepiece on any rig.
If I read your post correctly you did one of the options I was considering - put the "middle" part of the progressive lens as the "distance" part. That way the gap between distance and bifocal essentially goes away. I'm debating whether it's worth giving up the distance spotting of hammerheads etc (beyond normal video/photo range anyway) in favor of getting rid of the out-of-focus part of the near distance. Oh, well, all life is some sort of compromise...
I even thought about having two masks with me for that reason: clip off the distance mask and switch if I decided it was better at a given moment. Just what I need; something else to hang off my BC!