Hello,
I tried a 2.8/28 mm on my M 6/0.72. Even without glasses it is a little bit difficult to overwiew the whole 28 mm-frame at the same time. With glasses it is impossible. Furthermore the lines show more or less only the picture of a 30 mm rather than a real 28 mm lens.
The 0.72 viewfinder window in the M 4-P and later types was widened a little bit compared to the older types with only 35 mm lines. If you really want to shoot more wide angle you should use an external viewfinder or a 0.58 camera. But remember that the 0.58 wide angle rangefinder is not precise enough for a 1.4/75 mm a 2.0/90 mm, all 135 mm's, a 1.0/50 mm Noctilux, maybe even for a 1.4/50 mma at full aperture.