Just for kicks, I too google image searched for "space frame" and while most of the images were of entirely triangle-based space frames, there were in fact some that showed constructions using curved bars. So if we're using google image search as an authority, it's now proven that space frames can have curved bars. Notably, most of the curved bars I saw were in boat- or car-related designs, if you want to see the "proof" (though I would hardly call google image searching for something to be actual proof, but since it seems to be a metric you're using, I'll go with it).
