I could only try for like 10 minutes before going to work, but I found out that if you hold the stick towards a vertical wall, all vertical momentum is gone, so you can just stay in the wall as long as your holding the correct direction. I don't know if it works because it's a inclined wall (like, not 90?), but it worked with the left platform too.
I managed to play with my GC Controller so I'll use that to see if it feels similar to Smash games when gameplay gets more advanced.