Here's a dumb idea I just came up with: Attach a bright light that the computer can blink programatically. Write a program which: a) blinks the light, b) Blinks the light again 0.01 seconds after the camera sees the light blink, and c) repeat b a thousand times. d) Measure how long the process took relative to ten seconds, divide by 1000 to get your latency (including latency for sending the signal to the light, but hey, it's at least an upper bound).

Another thought that occurs to me is that people who work with music equipment (especially usb synths and such) care about latency a lot. It maybe worthwhile to hook into that scene and see what kind of insights can be gleaned. Here's a link to get you started: http://www.mathworks.com/help/dsp/examples/measuring-audio-l... Looks like they're basically using a feedback model as well.