I'm kind of surprised there hasn't been a mod to make the meter work in manual mode. Can anyone say what the technical reason is for this limitation?
There is no technical reason, but there is a reason: Canon wanted it to work that way.
The camera was designed primarily as an auto exposure, or one should say semi-auto as the user had to make one of the two settings, camera. There were only two reasons to set it manually:
1. You are using an auto-exposure flash or studio flash. In both cases the GN system is not usable.
2. You want to make an exposure at an f/stop different that what the meter thinks is the right choice for the ISO and shutter speed set.
In both cases, you don't need the meter to work. Turning the meter off when the f/stop is manually set gives the user a clear indication in the VF that the camera is in the "non-normal" manual mode.
If you want to take a meter reading and then shoot manually, its simple enough to put the camera back into auto-mode, read the f/stop in the VF, and then manually set the f/stop you want relative to the meter's suggestion.
I used these little beasties for quite a number of years and never found the "no meter in manual" behavior to be of any real hinderance. The trap needle exposure lock function served me for most problem metering situations, allowing me to leave the camera in automatic almost all the time. The situations where I used manual most were situations where the meter was useless anyway. These were mostly very low light situations with a mix of almost no light and visible light sources (e.g. street photography at night, bars, nightclubs, music venues, ...).
With the technology of the day, providing a match-needle type of manual metering would add significantly to the complexity of the camera (read: +$$). Turning off the meter is a far better indicator of being in manual than a simple flag of some sort. The various indicators of this type that have been used in camera VFs from time to time have a history of being overlooked and thus almost useless.
