I don't think hot air balloons are transparent to any significant degree: look at en.wikipedia.org/wiki/Hot_air_balloon for some example photos of what I mean.  Anyway, if I add just barely enough transparency to the balloon to allow a little bit of the inside light through (not enough to achieve the desired effect), I can already see the pattern on the opposite side of the balloon, as well as the horizon beyond, and I think that's very wrong.  :(

I did just figure out my ambience problem - my ambient color in Sky Lab was black.  Thanks for trying it for me, though.  (slapping self :)

I'm now wondering if it might work to put a solid, highly ambient copy of the balloon bag just inside the main balloon bag, and then add some transparency to the outer bag?  That way the slightly transparent balloon still couldn't be seen through.  And if I base the outer bag's transparency on an alpha texture that is less transparent in darker areas, perhaps it would prevent the "washed out" effect?  Does that sound like a workable approach, or can someone suggest a better one?