If you want to see a really good piston-filled design, look up Visconti pens. They came up with a piston system that lets you use a very large portion of the barrel, holding a LOT more ink. And despite the fact that they're Italian, they don't leak in day-to-day use (never flew with mine, so I can't say how well they'd handle the changing air pressure). See what you can find about it online - their pens are modern and NOT cheap. Montblanc on the other hand, well, my 149 (the famous big fat pen that looks like a black lacquered cigar) leaks like a sieve even when stored upright, capped. Which is a shame as I found it in the used 1/2 price case at Bertram's Inkwell with a left-handed Italic nib. And I'm a lefty.

I'd look into making the nibs with the 3-D printer. They've evolved from quills to steel and gold for a reason - the steel lasts forever, but it is stiff and scratchy to write with. The gold is not only beautiful to look at but it has greater flexibility, yielding a smoother, more comfortable writing experience. You might be able to get a compromise between them from the right blend of plastic. Quills wear out too fast (I cut one before, and while it works, it wears down fast and you have to re-cut it often - thus the origin of the pen-knife). You could also try glass - lots of pen stores, stationery stores and art supply stores sell glass dipping pens from Murano in Venice. They work well enough as a dipping pen, but you'd have to see what you could do about making a nib with a feed, otherwise they'd be a godawful mess.