Bud, your boot planner is just about complete. The Sheet metal guy got us hung up. (forming the pulley guards to protect your fingers) Good chance you will be seeing me and my dad in the very near future..
FYI the Keyser boot sole planer goes for $10K USD at last check.. And I don't think that includes the device to set the angle for the toe and heel ends. Currently most boot fitters use a sander and either freehand it or use an adjustable jig to cut this angle. Then they still have to adjust each one to match the lifter plates.
One of the failings of the bench jointer is the lack of power so they bog down, and they are quite noisy due to the very high speed the blade has to run in order to cut into the plastic. The adjustable infeed table, on the less expensive jointers are tricky to adjust and with the amount of vibration created by planing hard plastic they tend to move unless you really crank down on them. The result is often a bit of deforming of the surface. We found this out when we tried to mill off the surface of one of the commercial units to mill an angle into the surface.. In order to clamp it tight enough to keep it from moving, the table began to flex..
As far as the flat question on the Harb cant's I have a set sitting on my bench right now. (I was scoping out other options regarding the heel and toe angles) and when they are sitting on my granite reference surface, they do not sit flat (they rock diagonally). When I set them on my test boot sole, and screw them down they are better. They flatten out a bit, but they still cause the boot to rock a little diagonally. I measured them for cant angle and they pretty accurate but still not as accurate as a boot planed properly and with the flat lifters. Of course this is a sample of 1 of 1 (harb cant plates) and a sample of 3 planed and lifted boots so take my findings with a grain of salt..