One account I read, of problems with epoxy, in 'Sport Aviation', the magazine of the Experimental Aircraft Association, involved a person who was building an airplane using fiberglass/epoxy skin over a foam core. He became sensitive part way through the process, in spite of using all the proper protections; mask, suit, etc. He took a couple of weeks off to recover, suited up with special care, and went back to work. Shortly after removing the suit he went into anaphylactic shock and would have died if not for the rapid response of the local ambulance team.
I'll note that it's possible to become allergic to a wide range of things with repeated exposure. My violin making teacher eventually became mildly allergic to both maple and spruce. There are some things that almost everybody is allergic to, such a cockroach parts. That's why some people have a reaction to chocolate: it all has some cockroach parts in it. Better chocolate has less, and almost nobody is allergic to pure chocolate, I'm told.
Once you've become sensitized the amount of material you encounter doesn't matter. What's killing you is the response of your immune system to a perceived, but not real, threat. The hardeners used in epoxies mimic proteins, and that's what makes them sensitizing. I don't think that it requires a large amount of material to become sensitive, either; it's more a matter of persistent exposure over time. Not everybody will react in the same way, of course. Brian has gotten away with it for a long time. You might not. I had a neighbor who smoked cigarettes like a chimney all her life, and made it to almost a hundred. They killed my dad at 67. Statistically smoking is a bad idea, and neither is contact with epoxy that has not fully cured.
In terms of sound, almost any coating will have pretty much the same effect when it's as thin as Brian's epoxy fill. It's a matter of the stiffness and mass that's being added, and we're not looking at much here. Epoxy makes a good sealer in that it should cure reliably on most woods, is tough, has a nice refractive index that helps show the 'light' of the wood, doesn't shrink, and sticks to pretty much anything. There are other coatings that should work about as well, with less risk.