I had a neighbor that used to plant in stumps and would drag up old tree trunks and plant in the cavities. The stumps were left when they cleared the plot to build a home. I think the logs were found in the pasture. It actually made for a happy, careless but attractive, planting. Very informal. Primarily that was all done with oak and an occasional elm. If you hollowed out the juniper and made a drain hole, it seems to me you could line that hole with plastic and make it work. Wouldn't the plastic protect the potting soil from leaching?
A side note here. If you or someone else is good with a chain saw, cut the trunks into sections for plant stands. That's if you are going to be removing any. I was given a section and I've had it outside for years now. I wish I had a few more.
