There is no way to take the weight of the steadicam off the operator mid shot. In Russian Ark Tillman Buttner, used a custom Klassen vest and sat on a dolly for a rest, but he had the weight of the rig on him the whole time. Same goes for The Skyman system, Steadicams can be hardmounted to all kinds of vehicles or dollys, but that is a different setup to the operator wearing the rig and you certainly can't switch between the two mid shot. Maybe the shot could be broken into parts using some clever staging and camera wipes? As for lighter Steadicam systems, the Ultra2 is probably the lightest big rig. Tiffen make quite a few models, depending on the camera package you go with you could choose from several models. The Steadicam operator is going to be a very critical crew member on a project like this, I'd suggest getting an experienced op and working through your options with the guy (or girl) who is going to have to pull off the shot.
Brad