We've not tried OT, meds are working well for us thankfully, but have you talked to the OT about the meltdowns? Activity is supposed to tire out their body (& their brain), enabling them to focus better. Is it a quick reaction & then a calming down, or is it more of a drawn-out meltdown? Maybe the tramp is too intense for his brain/body & he needs an easier physical outlet, or working into a set period of time on the tramp. We love to play popcorn (you jump w/a beachball sized ball, when it hits you you get a letter-p, o, p, etc-til somebody's out), maybe a tramp-based game would cause less melting down.
Not advice on any of the above that you asked for, but I'm on page 107 of a great book, Parenting Children with ADHD, 10 lessons that medicine cannot teach by Dr Vincent Monastra, & I've already got 20 bookmarks in it. It's an easy read & very informative. He covers all points of view, giving them equal weight. I haven't gotten to a section on OT yet, but maybe it's worth checking out at the library.
Good luck to you!
*Jenny McCarthy has a group of Autism books & a website (don't know what it is offhand) but she's got sensory therapy dough on her website, as well as a bunch of other stuff... have you checked that out too?