You are getting into some uncharted territory. The problem with clear epoxy coats is that they get yellow and brittle with age, and the faster they cure the more yellow and brittle they will become. Being a petroleum product, epoxies have an affinity for oil, and the contact with our body oils from our fingers will speed the color change.
I see a bigger problem in that the Epoxy finish will be no better than what you would get from a waterborne lacquer. There has already been some information written here on using the product called "Enduro". They can be of similar hardness, and the Enduro is a lot easier to use than an epoxy; and doesn't have the same yellowing problem or the same affinity for absorbing oils.
Personally, I think that a stabilized wood is a better choice than using an epoxy coating on the wood. You are doing the same thing, only with a different plastic.
If you wish to try an epoxy coating, you won't find one on the store shelf that will not have the problems of yellowing with age and contact with body oils. There are clear epoxies where these are less of a problem. I am familiar with some of the System-3 products. You should give them a call, and ask for their recommendation. However, they take a long time to cure, and you will have to find some way to slowly spin the pen barrels while the finish is curing. Otherwise the low viscosity epoxy will all run to one side. A gentle amount of heat will solve the bubbling problems and speed up the cure, but heat will also lower the viscosity of the epoxy, and the spinning becomes even more important. The heated air makes the bubbles expand and rise more quickly to the surface of the lower viscosity of the liquid where the lower surface tension allows them to break through the surface and pop.
There is a woodturner in Texas named Steve Russell who has done some experimenting with epoxy coatings on pens. Look him up on the Internet to find an e-mail address. If you aren't in a hurry, or you can ask me and I will send it to you the next time I run across it.
