I think the At-om is designed to look pretty with it's tubes exposed. I don't know how stiff they are but I don't think it was the priority.
If you want the car to be stiff, the frame needs to connect the points where the coilovers are mounted efficiently. That means with a minimum of deflection and weight. You also need sufficiently stiff and strong mounting points for the suspension pickups, the drivetrain, any rollover protection and the driver's seat and harness etc.
Doing this basically requires triangulated pathways between the coilover mounts. We have some threads on this in the frame section of the forum. Here' a big thread,
http://www.locostusa.com/forums/viewtopic.php?f=39&t=2224. It discusses using software to measure your frame stiffness and includes some models for some members frames and also the Locost frame.For people that don't don't enjoy groveling with computers you can also learn a lot from balsa or other wood models and things like welding rod models etc.
It's a lot of work, but I enjoyed it. You can can get comments here if you post pictures of what you want to do.
Another thing to consider is that Locost style frame don't have to look like Super 7s when you're done. You can put a body on it like Jack McCornack did.
I think the midship format is open for some more designs, but I think the Midlana is a good example of a transverse middy. Look at Audi and Passat transaxles for longitudinal setups...