Maybe a sheet metal "band" around the trunk, several feet off the ground, would be enough to stop the scorpion from being able to make it to the treehouse. If the surface is smooth, the scorpion's tarsal shouldn't be able to find purchase.

Like a 12 to 18-inch wide band of smooth stainless steel or aluminum wrapped around the trunk.

Scrape x-inch bark off > apply metal mesh > 100% silicone sealant > butyl tape > wrapped aluminum > lock in with stainless steel hose clamps. Could even get a design lasered into the metal which might be cool.

Kind of like this from Reddit, but with those add'l steps: https://archive.is/yHYdj