Not sure if I'm making progress on this or not -
I'm running my new machine scaled at 1920 x 1200 instead of the native 1440 x 900.
Had an idea to try setting the resolution back to native and see if it scrambled. My thinking was if I change the resolution to native and it doesn't scramble, and then change it back to scaled and it immediately scrambles again, then I know I have a software problem and the culprit would be running at scaled resolutions.
Simple enough, right?
First, let's get it out of the way that the external display's behavior hasn't changed and I still get a white flash every time, regardless of the resolution the built in display is set at (note that the external runs at its native resolution). Note I tried both Thunderbolt ports and the results were the same, so I at least ruled out a bad port. Also, I have gotten the external display to now flash white upon restarts and shut downs, not just when going to sleep.
As for the internal display, well, it was almost that simple. After multiple toggles between native/scaled resolutions and multiple sleep/wake cycles for each toggle, I can confirm that it is much less likely to scramble at native resolution. Unfortunately, I saw two occasions where I still saw brief scrambling on the black screen prior to the login screen, immediately when the LCD backlight first turned on.
So basically the internal display scrambles less at native resolution, but still isn't perfect. Not sure if this really tells me anything or not.