Testing camera framerate variability by filming clock time
A binary count is going to be very hard to read in the image; if the exposure time is long enough to capture more than one displayed value, the reading will be very ambiguous.

I recently had a need to measure both shutter delay (latency) and exposure time for various DSLR cameras. I set up a microprocessor to drive an array of 40 LEDs (NOT multiplexed!) counting at a 10 kHz rate. The display is arranged such that each digit is represented by a column of 10 LEDs, where each column only has one LED on at a time.

100ms 10ms 1ms 0.1ms
   o    o    o    o  9
   o    o    o    o  8
   o    o    o    o  7
   o    o    o    o  6
   o    o    o    o  5
   o    o    o    o  4
   o    o    o    o  3
   o    o    o    o  2
   o    o    o    o  1
   o    o    o    o  0


Looking at the resulting images, it's actually quite easy to estimate both the starting and ending times of each exposure.

ADDITIONAL

A variation of this idea might be more appropriate for your application of evaluating the accuracy/stability of the video frame rate. Simply put the 40 (or whatever number) LEDs in a straight line (or maybe in a circle) and turn them on one at a time at 40&times; the nominal video frame rate. Viewing the resulting video directly will show you in real time whether the rate is correct and how much it is varying.