Hi, I don't know of anything that folds in half for 300 bucks. Like you, I got excited when I learned of the VAX 77 folding keyboard - what a great idea. My excitement quickly died when I saw the price tag. Talk about sticker shock!
VAX is an awesome controller I'm sure, but each unit is put together by hand in the United States. And therefore, with those kind of specs, the price has to go way up for them (Infinite Response) to get some of that time and domestic labor recuperated.
Like you said, I would be very interested in a name brand mass-produced MIDI keyboard (possibly with no sounds, just a controller) that still has 77-88 keys but folds in half. Not only is this good for flying, it would save my back from carrying a heavy midi keyboard (that does not fold) around to gigs. It's great that Infinite Response has the most sensitive keys (HD resolution) and optical sensors on the planet, but I would settle for 0-127 velocity resolution and a nice, light action that is fast enough for synth/organ playing and just weighted enough for piano playing.
NAMM 2012 was another letdown in terms of innovative piano products like this, as well as new modeling software/sample libraries. I guess we are not as big of a market as we thought.
Good luck in your search. Please post back if you get any leads.