It would take quite a bit of labor and trial-and-error, but I don't see why it couldn't work.
Perhaps you should start by buying drivers that have known values for enclosure volume that they sound best at. That way you have a goal for the stump's expected interior volume before you begin working with any wood.
The stumps have to be structurally sound; obviously they can't have rot. You would have to hollow them out, then coat the entire inside with some kind of water tight sealant or epoxy. Then you can measure the internal volume by filling it with water, and then measuring the volume of water. You'd have to be careful with draining and pouring in order to get an accurate measurement. Don't add too much at once, let it try, then measure the volume again. Through trial and error you can get all the stumps to have equal (or close to it) volumes by filling them with additional epoxy until their volumes are decreased to meet your driver's expected value.
The tricky part would be how to route the wires out of the stump.
Good luck.