Forgetful me! Fortunately, I slept on your question and realized I had described the solution in my how-to on converting G3F shoes to PE on the SM board, though I left out a step that hopefully readers would deduce for themselves. So here is your solution:

Model to the posed foot.

Use the fitting room to create figures rigged down to and including the toe, with your boot(s) as the object and non-zeroed PE as the goal, depending on whether you want to rig as a pair or separately. This/these figures will not conform properly.

In the pose room, zero PE. Your boots now have the analogous body parts to PE. Pose the boots to fit the zeroed figure. Export the boots as one or two objs with the body parts you will need from the thigh/shin/foot/toe chain. Reimport your obj(s) and fit them to zeroed PE.

In the fitting room, use the non-zeroed obj as the object and the zeroed PE as the goal. Add the bones only down to the foot. Do not/not rig the toes . Click create figure.

Et voila! Rigged boots