There are certainly macro bellows for the rz. I use the ones for the rb, they work fine with rb lenses. Med format macro is great fun.
As you mentioned, there is also a t&s gizmo that you can use with two short barrel lenses- I had the gizmo and sold it pretty quickly- not worth it. For homemade t&s the rz is less ideal than the rb because of the electronic shutter control, with the rb you can cook up all kinds of things.
6x7 format is so close to 4x5, most of us who do want serious t&s simply use 4x5. We also sometimes put the rb lenses on a 4x5 or 5x7 and get movements that way; Erie Patsellis has experimented extensively along these lines. Many of the rb/rz lenses cover 4x5 very well at moderately close focus i.e. portrait range. For high-mag macro, well you can use 35mm lenses on large format for that!
One final point that you may not know is that you can get substantial tilt effects at the enlarger. I have also experimented with bowing the paper to define in-/out-of-focus regions, it is easy.
P.S. David has it right, those who use medium format generally regard them as tools for specific tasks... they were never meant to be as versatile as a 35mm SLR. Hence in med. format we have SLRs, RFs, TLRs, view cameras...