I made it! It now works perfectly, and is also framerate-independent.
In the end I did go with an Array.
Play it online:
Download the CAPX (fully commented, for study):
And not only that, but I managed to solve a tricky problem that others have been having with jump-throughs: platform permeability from the sides.
until Ashley makes an awesome Jumpthru behaviour that can distinguish which side you're entering the platform from (please do <3 ), my solution is simple:
make a "JumpThroughProtector" sprite, one sprite wide, and as high as the platform you want to protect from a jumpthrough from the side. Set a few conditions, and now you have a platform that you can jump through, but will also act like a wall if you're pushing against it when not on the floor.
You can check it on the two small platforms to the right: try jumping against the white vertical stripes. You can't.
Now walk under one of these platforms and jump up. Works like a charm.
This is preferable, in some cases, to switching the Platform from solid to Jumpthru, because sometimes you may not want the other players to exploit a platform that should only be jumpthru-able for you, since you're the one who's jumping the right way.