Tony MacAlpine also covers legato playing in a couple of his videos - their titles escape me, but any of the fusion players you can scoop some knowledge from will certainly help - another one Frank Gambale - the king of sweep. His video will almost have you give up music altogether !! Check it out online in a video search.
That Frank Gambale video is awesome (if really incredibly dated...that man should be taken out behind a barn and shot for wearing those clothes).
There is one very important caveat emptor surrounding that vid, however. The technique he uses is HIGHLY PROPRIETARY. Nobody sweeps like he does. It's a great technique, and it IS accessible, but once you get really deep into that style, it's very, very difficult to get back in the groove of "standard" sweep picking, or even alternate and hybrid picking.
Gamble even pretty much tells you as much in the video itself. The thing is, though, it's such a fluid, beautiful technique that it SOUNDS like legato playing, even though it's not, technically speaking. You're right, this is not a standard video. It will discourage all but the hardiest of players beyond belief. He just makes it look WAAAAAY too easy. He's so laid back about it, he looks almost bored. When he goes "ok, so now let's play it up to tempo", and he rips through approximately 6.5 billion perfectly and beautifully executed notes in about two seconds, it makes you want to look down at your guitar and sigh in frustration. Just goes to show, now matter how good you are, there's always someone better than you.
Curse you and your absolutely flawless technique, Mr. Gambale!