Dave, I made the same mistake on my project, but only on a sill plate. I think Andy had a good suggestion fiber glass resin or maybe epoxy, sounds strong.I took a piece of oak 1x and drilled through a 1" hole then clamped the board to the tenon at the right location and drilled a new hole through the tenon with a hole saw. You now have a hole that is oblong by 1/4" I then took a piece of 2" thick oak and layed out 2 1" circles with the center lines at 1/4" apart. I drilled a hole through the first circle and then with a band saw cut around the outside of the line of the second hole which left me with a plug that was 1/4 moon shaped. I glued the plug in place with polyurethane glue and held it in place with a slightly undersized peg until dry. This worked for me and after 9 months the joint is still tight. Hope this helps, Good luck, John...
