Steve - I'm not familar with a "newel post lock". I've seen the post plates...could you PM a link if you know where I can could find them (I tried Googling "newel post lock"...not much came up). I've also seen some installs which use a couple of threaded rods to secure the newel post from the top, though I think I'd need more vertical space than I have to lift it off. Another install used a hollow newel which was bolted from the inside and you needed quite a long extension for your socket wrench to get to the nuts.

Thanks for your help.