I found your site by searching on "glucosamine" and "tae kwon do"; I'm a middle-aged tae kwon do student (7th gup, after about 1 year or so of classes) with crappy flexibility, also trying glucosamine. I'd be interested in what you found. I don't see your follow-up. Did you post one?
In one of those little life coincidences, I was looking at my bottle of glucosamine and thinking about writing a followup entry about an hour before I saw this email. Weird.
At any rate I have to say this: my flexibility definitely improved after taking glucosamine. This wasn't a scientific test, but it seemed to work for me. And this is was after seeing a big improvement in my flexibility which seemed to plateau.
When I first started taking Tae Kwon Do two plus years ago I could barely touch my toes. My flexibility gradually improved, but the improvement seemed to stall. The glucosamine did seem to help, and today I can sit with my legs out straight, cup my hands beneath my feet, and put my forehead on my knees.
(Submitted for cross-posting at Karate Training)