Cut the mortise with a Forstner bit so it is as long (or even slightly shorter) than the tennon. Make a pattern that mathes the mortise, lay it on the square tennon. Mark the curve on the tennon, and pare fairly close to that line with a sharp chisel. Sand to an exact fit.
Or, make the tennon slightly short so it's maybe 1/8th short of protruding. Then cut out a shape you want to showWITH the grain from a piece of your stock and shape the edge of this piece with your roundover or other bit. Shave away a small bit of the edge to fit in the leftover mortise depth. This will give what looks like a through mortise, but it will be with the grain on the end of this cap. Glue this small cap in place, it will probably last 200 years and look better than the end of your tennon will ever look.
The width of this cap will determine what it looks like on the finished piece. The slight tennon on the back of this cap can be cut to exact width on your router. Remove the shoulder on a long piece so it's uniform. Chop the tennon to length with a Japanese saw and a chisel. It's all hand work, but I don't see any way to avoid the hand work. This cap will also hide any wedges you might wish to put in to make a perfect tight fit.
However, NO ONE will ever check the length of these caps for uniformity and fit. The glue is the hero in this tale.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 03-04-2019 at 01:10 PM.