Hey Guys,
One thing I found out the hard way in regards to threading a hole in a wood base plug is that you need to use face grain wood (as you'd cut from a board, and mount on a face plate, if turning) as opposed to the end grain wood that you'd use if turning a standard plug. I ruined a couple of really nice walnut plugs, and one of cherry, a couple years back trying to do just such a thing on an artillery horn for a buddy of mine. After stumbling upon this bit of info, I looked back at some photos of original artillery horns, and literally every one that I could discern used face grain for the plug. It bears mentioning that a tenon turned of end grain, as you'd use for the knob-like piece that screws into the plug, will thread just fine. Just something about cutting threads in that inside hole that likes to tear out. Using face grain on the other hand, allows you to cut with the grain as opposed to across it, and will produce very clean threads, provided, as Tim said, the tap is good and sharp. Hope this is of some use,
Mike