This is cool. I'm assuming the custom block can be given a model and textures with a resource pack? (I'm not very familiar with those.)
The only downside with this is that it can't be contained within a world directory (so it can't, e.g., be used on Minecraft Realms).
Yes you can use the resource packs to model and texture, that is my preferred method.
Resource packs can be put into map files, I think the folder name is "resources" or something, idk. If you mean the mods, yeah that'd be nice. Mojang is supposed to be working on something like that... so they say