Currently the character toolbar can only be read by generating some save data in memory.
That means the more data the world has the longer the call will take, which can take seconds to execute on some worlds which translates to freezes.
It's also not the kind of thing you call once and forget about it because it's data at that point in time, so it needs to be called frequently to be aware of changes (what are in the slots, selected slot, etc)

It also doesn't contain any info about the page you're on and that's quite confusing.

Additionally you won't see any icons because mods can't draw textures dynamically, they require a transparent material definition which means I'll have to take every block's icon and make a definition from it which certainly won't include mods... and from what I tried the transparent material definitions can't be added dynamically via code either.

So... basically it's not worth the performance hit and effort to get a crappy result :P