I won't pretend to be any kind of expert on weapon-fighting, but I've given the idea of a pen as a weapon a lot of thought over the years. So a couple comments. Jeff, I know that you are more than well-aware of most or all of this.
One of the things they taught us in the Marines was to make use of expedient weapons - in other words, chairs, shovels, helmets, etc. A pen is an expedient weapon. It is not designed to be a weapon, so by nature, it is always a compromise.
Expedient weapons are not traditionally (except in the military) seen as fight-enders. They are intended to provide initial self-defense, create space, and allow one to either LEAVE or go to a more lethal weapon if required. Not to say you can't kill with a pen; but that really that is not its forte. I would not feel empowered to remain and fight on when I could escape because I had my 'combat pen' with me.
The way to proficiency with expedient weapons is the same as with purpose-made weapons. Practice. I know quite a few people who 'know' how to use a Kubaton, but they don't. They had some training on it, once or twice, and now they carry it. Use it? Theoretically, I suppose. Might work. Might not.
In my experience, purpose-built weapons that are supposed to resemble other things are seldom as easily-missed as the owners hope. Pens built by self-defense companies are for self-defense, not writing, and most law enforcement and professional security people know it. More importantly; prosecuting attorneys know it. If prosecuted or sued, you can be certain that your 'pen' will be seen by the opposition as a purpose-built weapon, the same as if you carried a knife or a firearm. You're not fooling anyone at that point. A pen that is a commercial item and sold as a pen and nothing else will simply not be subject to the same level of scrutiny.
Obviously, a dual-purpose 'pen' that is built to be a self-defense weapon is going to be better as a weapon in most cases than a generic pen with no inbuilt self-defense design. That is a trade-off that one must consider when making up one's mind about what to carry.
As to the pen itself, I would consider these things...
It must be rigid enough to penetrate without breaking for a period of time.
It must have a friction-grip that resists slippage.
It must be instantly deployable; no cap to remove, no buttons to press.
It must not leak ink in my pants pocket.
How is it going to be used? I can think of three ways. As a push dagger (extending between the fingers, back to the palm at the wrist), as a stabbing weapon extending from the top (thumb side) of the fist, and as a stabbing weapon extending from the bottom of the fist.
If I had to guess, I would think that using the pen as a stabbing weapon extending from the bottom of the fist might work best, although it seems somewhat counter-intuitive. One has to use one's fists as if delivering a hammer fist (tetsui). But this has some advantages. First, one might be able to use the thumb to keep the pen from slipping through the hand when making a thrust, and second, it is after all a tetsui, capable by itself of doing damage. It has the additional advantage of being a bit more able to deliver as an upward or downward blow to the soft tissue of the face; holding it the other way pretty much limits you to uppercut-type strikes to the facial area.
So, if I were going to be carrying a pen as a self-defense weapon, I would look at the available common and commercial pens; not those made by self-defense companies under the rubric of 'pen'. I would practice with it as intensively as I do my bag work or basic exercises in and out of the dojo.
Oh, and you can practice safely with a stabbing weapon such as a pen. A short length of garden hose works well with an uke who is wearing some padding. If you don't mind getting dirty, a little wet paint on the end will show where your hits are on him or her. Remember of course that he or she has to defend to the best of their ability, and if they stop you, they stop you; consider abandoning that tactic.
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