A vent pipe to the roof in a Hi-Lo would probably much easier vs. in a TM, since it could be a hard telescoping pipe.
And of course, whatever solution they do come up with, particularly if it is some odd contraption that vents to the roof, the solution has to be acceptable to potential buyers. You don't want it to tank sales. Imagine the sales demo where they show you how to connect the vent to the poop tank? After all, they do already have an acceptable solution to many people, that being a cassette toilet.
A small diameter version of a sewer slinky hose might be an option, particularly if it was hidden or disguised if inside. Stretched tight, it would be straight and less unsightly. If outside, I wonder if it could it be run up the shell support arm, much like the wires for the A/C, lights, TV antenna, etc.? That might be be workable particularly if they used a traditional black tank mounted under the trailer instead of the Sealand toilet. I know there's not much room under the trailer though.....
I wonder also if they could run a vent pipe up inside the wall of the base shell with a flange or coupler at the top of the base shell that would join a similar flange at the bottom of the rear shell when the rear shell is raised. Another pipe would be inside the wall of the rear shell, which then vents out the top of the wall before it hits the roof (which would meet the 6-foot requirement). Two elbows would be required where the top and base shells meet to do this, and some foam seals on the meeting flanges could make for an adequate seal for such a low pressure connection. Even a magnet seal could work -- there are magnet seals used in dryer vents, which is a much higher pressure application (
https://www.magvent-dryervent.com/). Some sort of snap locks could also be used at the seal to make a pressure fit once the shell is raised, though the risk there is that an owner would forget to release it before putting the shell down, and cause significant damage.
Dave