Tony,
Tung oil has been used on pens by a few since almost as long as this forum has been operating. It isn't used regularly though, IMO. Tung oil needs to be studied closely if one wants the best that TO has to offer as a finish. Some finishes that say "TO finish" have little or no TO in it. Most "100% Tung Oil" finishes need days to cure/dry for a single coat. Successive coats take as long also.
TO is a super finish and highly water proof, better than any other oil as a water protector/proofer, but length of time for curing is a factor. There are some TO's with dryers/polymers to help them catalyze faster, but beware of "TO Finishes" . . . look at the specific contents. With other TO finishes, read the contents and specifics for which that particular brand and or blend was created. Even then, TO will not give a CA type of shine, but you can get an almost satin sheen that will make some woods look luxurious - but again, it takes time and work. If you like wood look without the shine, TO is excellent for protecting the wood.
Polyurthane can work but there are different kinds of poly. The kind that goes on gym floors or bowling alleys will work well. And applied over TO should work well.
One thing that needs to be pointed out with pens to those new to pen making - pens made from wood blanks are not the same as furniture. Furniture and wood walls do not go through the same handling process that pens do. Pens are handled a dozen times a day, placed in caustic humid hands and pockets, often put into pants pockets or purses bouncing around with pocket change or stuff in a purse. Placed on a dash or car seat where it can reach 150° or taken outside in a pocket when it is 20° and taken from a controlled environment to huge swing in humidity. This takes a toll on finishes. Wood furniture does not go through this and the finishes for pens should not be considered to be the same. A caveat is that owners of fine expensive pens will take care of their pens and clean them on a consistent and regular basis. Most people don't do this. The caustic sweat of hands and shirt pocket humidity will deteriorate most common poly type finishes kinda quickly.
BLO has been mentioned and it works for many. Watch out for applying BLO on oily blanks such as ebonies and a few others (Oily woods) as it can easily interfer with adhesion in those cases. But for non-oily woods they do fine, but not absolutely necessary*. If you are in a very dry and warm/hot locale, BLO may not be necessary for speeding up the curing process.
*As to "not absolutely necessary" BLO does well for many in many instances, but down the road, many people do run into finish problems. The conclusion is that "the more things you add in, the more chances for problems". Wet sanding, BLO, Accelerant, DNA (Denatured Alcohol) all help, but sooner or later there will come a time when it just doesn't work in a particular situation. Then the answer is reduce to the simplest form of finish. And this is a learning experience. There will be times that BLO works fine; There will be times when wet sanding will work OK; There will be times when accelerant will help tremendously. And the answer is: LOOK at all the different ways you have learned, and know how to use different ones in different situations. No one formula works ALL the time for the majority of people.