I use this simple device when planing the edges of long boards. It's especially useful when the board is too springy to be clamped between a bench dog and a tail vise. I hold the board in the v-shaped notch with my left hand and plane with my right. Use 1/2" or 3/4" plywood so it doesn't split.
Ping Sing Li
This is a really cool idea. I think, though, that I'd make the notched piece out of two or three thicknesses of plywood. That way I wouldn't have to steady the board with one hand while planing with the other. After all, we use holding devices to keep our hands free, right? And who wants to try planing the edge of a long board with one hand?!