The camera port gets just one eyepiece (the left I think?). The trinoc setup has an angled mirror that slides into or out of the light path and another behind it that reflects the image back upwards, really a pretty simple setup.
There aren't any real stereos that do that kind of side-by-side thing as far as I know. You can get fancy forensic dual scopes that will put two views side-by-side or (within the b&l family in particular) modified stereomicroscope stereoscopes which were used for viewing stereo pairs of aerial photographs. Forensic scopes will likely be able to translate that to the camera, but I don't think the stereoscopes were ever made trinocular--not reason to take a photo when what you're looking at is already a photo I guess. I have been very curious about these stereoscopes though, particularly the b&l Zoom 500, but never had a reason to get one. Maybe once I've cleared out more extra scopes here. The best solution would be to get a Nikon SMZ-10 (the original, not the SMZ-10a) which allows you to switch which light path the camera pulls from with the flip of a lever.
That's interesting about the eyepiece angle issue between the sz7 and balplan. Another interesting item you see with those b&l stereoscopes is a convertable "mono-stereo" eyepiece, which I guess has something to do with that particular issue. I've never bought one though.