Well, a few measurements and a quick sketch tells me that my plan isn't going to work unless I can get the pinhole closer to the aperture in the shutter plate. For anyone who might want to do such a conversion, the easiest way to this it is going to be to insert the pinhole plate from the front of the camera, into the very bottom of the conical well; short of mounting it directly to the shutter plate, which would require removal of the shutter mechanism, that's as close as you can get and it should prevent vignetting. For the moment I think I'm just going to reassemble the camera in its original form and use it that way. I'll come up with another pinhole project for the short term.
Thanks for all the help.