What about the other way?
I admire your goal and can only imagine how hard a ride up Haleakala would be. But the funny side of me thought "why not just ride it down?". A whole lot less training required!! :-)
I did the organized beach cruiser ride down years ago and it was both interesting and boring. The moon-like scenery of Haleakala was amazing, but a cautious, organized ride down was slow to the point of boredom. We were sorted by size, with bigger guys at the back. I was near the back and pretty much rode my brakes the entire way, versus screaming down the mountain like many of us wanted to. My small wife was the first person behind the tour guide and has a completely different memory of the ride. It was also cool (no pun intended) that we started in long pants and heavy jackets at the top of the volcano, and by the time we got down to Paia we had stripped all that off.
I've heard that the cruiser rides down the mountain don't exist anymore but not sure if that's fact or nonsense.
At any rate, good luck to you. Post pictures!!!!