I just recently received my TWSBI Vac-700 Demonstrator in Medium, and although I am leaving already (oh bittersweet, to receive a pen and not use it immediately right after) from Japan to California, I'd thought it to answer my own question:
Can the TWSBI Vac-700 truly withstand leaks on an airplane, be used on-board, and not leak when landing (or even when in use)?
To answer this question I've filled my TWSBI with bottled water (so that should it leak or heaven-forbid, explode) to the brim using the technique shown by Brian Goulet, and will have it in my jacket pocket nib up, then when the airplane reaches a constant altitude, I'll begin scribbling first with the water inside the feed with the blind cap closed until it starts to dry out, then open the cap so that water flows to the feed. Hopefully nothing happens other than continuing to write. After about half of the water has been used, I'll close the blind cap, cutting off the water flow, then place it back in my pocket again nib side up. Once I land, I'll check my pen, if there's no water, then we''ll know this pen is perfectly suitable for Airplane carry and use.
I'll let you know the results as soon as I get home, and take pictures if possible.