I model furniture and have done material stretched over a tube frame, your best bet to get the flat pattern is going to be making the tops out of sheet metal, and then flatten.
TO have the frame driving the fabric dimensions you are gonna have to decide a lot of the smaller details ahead of time - but i certainly think its doable..
I used to do something like this. I always modeled the desired shape of the fabric first then based the frame around that. I'd shell the fabric shape by half of the thickness of the tubing and then convert those interior edgse to 3D sketches to run sweeps along to get the frame. Does that make sense?
That might or might not work for your purposes, it sounds like maybe the frame has to be the driving force here.
modeling a fabric top that is stretched over a frame should not be a problem. Having a convertible top that actually folds and acts like fabric during motion would be a bit more difficult but I still think do able. You will need external references at points to "link" the top. If you simply wish the tops to be stationary you will just need to their geometry for things like sweep paths or to define surfaces that you later thicken.
Do you have a simple example you are trying to work with? Maybe that would be best to help out.
Have anyone tried making the flattened pattern with Logopress?
When i draw ud an open tube with a circumference of 3620mm the flat pattern is 3500mm long. Could be a good reference...?