I used an Agfa B2 to make a pinhole, and had the same set-up. I put the pinhole where the lens was and got a complete circular vignette. I haven't tried changing it to the front yet, but I think it's going to vignette in either spot. (How's that for an unhelpful answer?) I'd be very interested in how yours works out, just to satisfy my curiosity. I'm sure someone is going to come along and throw some serious math at this question and give you a real answer.