Yes that's the stuff,flowers from the top of the cactus are sugary and can be made into jelly.November through spring till it flowers ,is the time to pick it.In the texas hill country it grows wild and is often used as cattle feed.Ranchers burn off the needles with torches called pearburners and the cows follow.I would think in the secondary ,I could add about a half gallon of pulp.
How intense is the flavor? You could start with 5 pounds and see if you get any of it - if I remember correctly it kind of resembles a green bean in taste (nopales). I don't think I'd want a green bean flavor in my beer but hey, go for it and let us know how it comes out.