Here is the tree after the cut, but still in the jaws of the workmate. I removed about 1 1/2" of the bottom of the trunk (in two 3/4" cuts).
Well, at least I'll be able to figure the age of the tree!
BTW, I've made a note to Santa, asking for a bigger workmate! It was a bit tricky getting the tree into the inner part of the jaws.
Of course, it probably would have been simpler just to take the tree over to a friend who could hold it for me
This cut was made August 1st.
Thanks to Jim Smith and JVL, who talked me into it. If I contemplate doing this on future trees, I will scar them below the new soil line and dust with rooting hormone before potting down in order to get more roots closer to the surface.