Hi Gary
I have never done this...just a thought when you stated it is to be heavy. The extra weight would be in in the barrel, the lower end of the pen. These pens do not post, so as you hold the pen to write with, the extra weight would be distributed between the finger grip (typ:two fingers and the thumb) and the end of the pen resting on the index finger knuckle. I just tried adding some external weight to a Jr Gent here at my desk, holding it in writing position, and it seemed to be balanced in my hand. This concept is to add a thick walled tube-shaped liner inside the barrel only (bored to accommodate the ink cartridge. It could be added later if the standard config of the pen is not heavy enough, needing onlyto knock out the end cap to insert the weight. It just depends how heavy your customer would like it to be, and how much extra work you are willing to put into it.
Steve