Layman. Today I put a piece of wood besides the work and both clamped in a workmate. This piece of wood stood proud of the work, and upon it sat the rebate foot of my electric planer. I was surprised to see that the planer also planed the piece of wood. I see because the blade in the planer sticks pround of the base-plate of the planer. I think this is natural occurance when using the rebate feature and nothing can be done to avoid cuting the piece of wood unless the blade matches exactly with the edge of the base-plate. And even then I suspect there would be some cutting as I used the planer.
I put my mind to homebrew a feature bolted on my workmate that used angle iron for the rebate foot to rest upon, but I don't think this will work. I'm a bit dissapointed.