I don't think this will be possible for X-plane. I'm an OpenGL programmer and have experience w/ the X-Plane SDK. The only way to make this work would be to have it non-stereo, which is most of the fun.
The main issue is we do not have control over the full x-plane render loop. That means we can't render two separate images for each eye, but we'll have to use the same image for both eyes.
A non-stereo version would still work, however the resolution of the dev kit rift would make it _very_ hard to read the instrument panel. Also due to the lack of spacial awareness in the rift, you could not do certain head motions that one would normally do with a TrackIR such as lean forward to get a more clear image of your instruments.
When I get my Rift I will experiment w/ creating a non-stereo version. Maybe the FOV and simple head motions will be good enough to make it enjoyable to fly VFR.
Incase you're working on one right now, I'm curious to see how you'd implement it? I plan on using a PBO to read from the current FBO and use it as a texture, then draw it onto two half domes per the Rift's documentation. I could map the FOV by assigning it in the x-plane settings and hopefully not get too much distortion.... any other (easier) ways you can think of implementing it?