I use bar soap, and it's always been fine. I'd caution you against using that wax ring for anything but installing a toilet, though. Having done that job a number of times when remodeling, it's some of the stickiest, nastiest crap there ever was- if you can't get it off your fingers, I can't even imagine how hard it would be to get off unfinished wood! It's also going to pick up every speck of anything that may be floating around the shop as soon as you open it, so expect it to be a real mess unless you plan to keep it in some kind of covered container after opening.
Perhaps a better experiment would be paste wax. I think I may have used it in the past for this, but I can't recall how well it did or did not work.