It's basically impossible, the only way this was implemented previously was via SpoutCraft, which is a highly outdated, and abandoned (once again) plugin. On that note, I don't think MeepCraft is planning on developing it's own API similar to it (if they are though, hats off to them).
If it were possible +1, like I said, closest plugin to it (in "vanilla") would be AnySlab, which only makes ghost blocks or AdvancedSlabs. Both of these are almost purely admin tools and couldn't be configured for common use.