Together with the Pixel Qi you make reference to, there are a number of display technologies available or partially developed for outdoors use. And some are relatively old: has nobody here ever worked with, eg., a Palm Tungsten with transflective display?

N900 had a transreflective screen, officially as a selling point.

I think my Surface Go first gen also has it, even when not advertised as such, because I can switch to white themes and almost work with back-light off in the direct sun.

I always wondered why we couldn’t have either a lens at the top edge of a laptop display, or a translucent case to let the sun shine in behind the screen. Maybe the colors wouldn’t be perfect but you’d get more light behind the screen no?

Find yourself a half-silvered mirror, stick it in bright light, and put another mirror with some kind of diffuser behind it.

What happens is that the light in front of the mirror overwhelms the light coming from behind it. I think you'd pretty much have this problem with any kind of transmissive display, whether it's internally illuminated or relies on transmitted light. Unless you can also shade the front of the display, you'll see little.

Early box cameras, the type that had a drape over the eyepiece or viewplate for the photographer to compose the image, also had this problem.

All of these rely on obscuring some portion of the transmitted light to create a visible pattern. That's always going to be muted by ambient light from the front of the display. It's why we watch films in darkened cinemas but read books (reflective medium) in brightly-lit libraries (or at least with a strong reading lamp nearby). Note that with any LCD-based display, the polarising layer itself blocks ~50% of the transmitted light.

This is why high-ambient-light displays strongly favour reflective media. Ink on paper, paint on canvas, billboards. E-ink happens to be a rapidly-updatable version of this, though not quite as rapid as the electronicly-modulated pixels of an emissive display.

What would be a game-changer would be a material (or suite of materials) which change colour under some rapidly-switchable field (probably electric or magnetic), preferably either on the RBG or CMYK colour space, probably from a white baseline. A response in the 16--120Hz range would be sufficient for video. These would produce the effect of a printed or painted display that could present high-response video and which you'd illuminate from the front rather than back.

Though I think I'm already shuddering to consider the possible applications....