While the photodiode gate concept seems simple in principal, it is quite difficult in practice. The contrast ratio of the display segments between on and off is not as large as one would like to get reliable switching even if the photo detector was placed right on the display. When I first got the Insight, I played around with trying to get a single led to light up when the MPG bargraph went into the lean burn zone. I mounted a photo detector right on the display, I also tried a plastic fiber optic to bring the light into a dark box where the photo detector as was coupled to it. When the sun got onto the display it caused the led to light even when the segment was not on. Threshold adjustment was needed between day and night even without the sun on the display. A single background light compensation photodetector was able to help, but unless it was placed right in the area of the bargraph segment that I was looking at, it could compensate incorrectly.
Detecting the 2 segments of the 100's digit and the 7 segments on the 10's, units, decimal point, and 1/10 digits would be 24 detectors that would need threshold adjustment and compensation inputs. The only way I can see to get the detectors to see the display without being in the way of your view would be to use a lens to image the display onto the detector circuit in your flat black box. The position of the box/detector to the display would be very critical, and difficult to adjust..
You would be basically making a custom 24 pixel camera. The $50 video camera already has AGC, light compensation, 640X480 pixel detector and video driver built in, alignment is easy since you can see on the video display what the camera sees.
Geoff Shepherd suggested that the video image could be sent to a computer where OCD software would decode the numbers, and then one could output 7 segment data through an I/O card. While that could work, it is even more complex .
I guess I don't see what is so hard about buying a camera and monitor, pluging it in and adjusting the lens for the view that you like?
The video solution is pretty much two off the shelf components, and a little work to mount the camera and LCD display.