So this thread got me to thinking earlier this evening.
A while back I purchased a small Wally Frank dublin (marked J.R. Special 14) - a lovely little estate, save the stem was a bit loose. Interestingly enough, the problem was a slight bulge in the middle of the tenon. That is, inserted half way into the mortise it fits tight, but when fully inserted is a bit loose. Because the heat-and-expand method was not an attractive option for this particular situation, I asked my wife if she had any clear nail polish. Sure enough, she did. Applied a thin coat on the thinner portion of the tenon, let set, and voila - perfecto.
Smoking some Peretti's Oxford Flake in it as I type.