Check if you can manually set your 3D TV to the type of 3D input it's receiving. A quick look at one Sony 3D TV manual showed it could be manually set to generate the 3D image from either side-by-side or over-under images, where the content for the two eyes are provided in one extra wide or extra tall image.
It's possible if you can create a frontbuffer of the right size then you can just render two images in this way and set your TV to look for it (seem to recall this might be how Avatar the game did it on the 360). Your content would then potentially be rendered correctly, but the 360 guide or pop-ups that get overlaid by the system would be displayed incorrectly until XNA officially supported 3D.