I don't think the duration of the flash is an issue. The sequence of events is as follows:
The camera will be set to its flash syncronisation setting. This sets a shutter speed of either, 1/60, 1/125 or 1/250 of a second. (I believe Nikon can use 1/400 but I'm open to correction).
When triggered, the focal plane shutter curtain opens completely.
The flash is fired
The shutter closes.
In order to achieve correct exposure, the camera's aperture setting is the critical setting.
A flash meter, I reckon, should, ideally, be able to trigger the flash setup itself, and monitor the light level received during the camera synch time. I would think that a PIC could work here (I have no idea about programming though!) and would simulate the camera operation to "open" the shutter, fire the flash, "close" the timing period and calculate the amount of light received and display results on a LCD display.
As to how to design and program something like this - I am at a loss, which is why I'm here!