Yeah, the timing is a bit difficult; I can't do it 100% of the time, yet. I don't have the game in front of me at the moment, but IIRC, you should hit L or H a few frames after connecting with the second H. I would post a vid of the juggle being performed, but my PC w/ a capture card is packed up at the moment. It's easier to connect with Energy Field, so try it with that first, just to see if it works. Practicality-wise, it may not be a great idea, since you use up the entire super meter for like 2 extra points of damage.
EDIT: It seems to me that combos aren't as exciting in this game as I thought. I think that the most damaging combos for everyone in the game is: ff+L, (L+B), pause, unfly H, ff+H, the character's personal string, and then super. The only exceptions are Piccolo, Krillen and Majin Buu, but all they have to do is replace unfly H with unfly L. I suppose with Super Cancel you can do more interesting things, but otherwise, that's the best you'll get.