...So I envision ever-increasing sailplans requiring
ever-more-massive rigs to control them spiraling ever-outwards until the
physical structure of the vessel itself is incapable of containing them, all
just to travel slower than a rowboat.

I think you're right. I made some napkin calculations (and did a
little mitchlet modeling) on this idea seven years ago and came pretty
much to the conclusion that you'd need to do something pretty radical
to make a true swath cat move in light air. The wetted surface is what
really does one in. A friend of mine did a compromise design with very
long bulbs down in New Zealand and he was happy with her, but she was
always a motor sailer and is being refit to be a mostly motor whale
watching platform. The real advantage of swath is that it makes sea
kind boats. At some speeds they may also be efficient but only where
wave drag dominates and only if you can get the displacement far from
the surface. I've noticed with the local swath boat (Navtec) that they
create a pretty energetic wave train, but she's a comfy boat in most
weather. As others have noted you'll also need to think a bit about
stability if using sail plans that have a heeling moment (really big
kites might work). Foils plus some kind of reserve bouncy (ie. hulls
near the surface) seemed likely candidates to me but both add drag.
There were lots of other issues with making the thing work and making
it maintainable (eg. access to the hulls, control surface linkages
&c.). And it would be a bugger to build and expensive... Seems like
the kind of boat project that keeps folks from getting out and
cruising...

-- Tom.