Even easier would be a closed end tubed pen with liquid in the end... But i feel ya on the liquid center barrell! Reminds me of the "undressing woman" pens from back in the day.
Your idea is really cool, and even possible in theory. I think i would cast the tube in a hard wax. Then turn the wax to the PRECISE thickness. then recast the tube in PR. Then drill two tiny holes in the brass. heat the wax and add air pressure.
What PR are you using? Silmar 40 (I think) can stand up to higher temps which might be handy for wax removal. Ask around the casting forum... someone will point you to a higher temp material!
Something else to think about.. On a Jr. Gent, if i am not mistaken, the barrell is ever so slightly tapered to a smaller size on the nib end. I would replace the nib end coupling thingy with one from the rear end of another Jr. Gent, or either end of a Jr. Emp. That would give you a uniform barrell thickness, and the a tiny bit more material on the nib end for strength. That odd sized coupler on the Jr. Gent is fully interchangeable with the Jr. Emp, and either end of the Lil Gent. SFE.
I sure would like to see your results, even if it fails (the first time)!