There is a plugin out there that turns your "Pose" to the "Elytra Flight" one, where your 1 block tall. Perhaps something working off of that same code with some tweaks could work to allow "crawling", with all the functionality and visuals that entails.
As for climbing, I am less keen as that means staff have to try to rework all the builds that are meant to be blocked off from rando noobs who like to edgy parkour and act like its IC movement to get into places like, the guard bases, etc. Which would mean spamming a lot of barriers, probably giving barrier walls a lip in both directions to prevent climbing...
Honestly that might not be too hard, but making it work from a code point of view is a bit finicky.
Perhaps being able to rightclick the sides of some blocks to launch yourself up slightly, with a delay of say, 3 seconds, would work to simulate this? Make it only work on blocks that COULD be climbed, IE Cobble, Brick, stone brick, Logs, etc. Glass and quartz is a no-go.