So just to be clear, you’ve tried the following steps in this order?
1. Use System Preferences to find the resolution of the projectors.
2. Create a jpeg that’s just full white at the same resolution as the projectors.
3. Turn on the computer BEFORE the projectors.
4. Open Isadora into a Scene that’s just a Picture Player actor outputting the white image to an unmapped Projector actor.
5. Show stages BEFORE turning on the physical projectors.
6. Turn on the projectors so that the first thing they “see” upon waking up is the full white image with NO mapping at their native resolution (to prevent them from doing any auto-scaling).
I swear the problem you're having sounds like the exact issue I was having a few years ago and the steps above solved it for me because it turned out that the projectors were auto-scaling/auto-keystoneing and the only way to prevent it was to send an unmapped image at the full resolution before I turned on the projectors and letting the projectors "wake up" to that unmapped white image.
Can you please give us the make and model of the projectors? (Also make sure to go into the menu settings for each projector and ensure that you’ve disabled anything like “auto-keystone” or “auto-scale”.)