I mostly use MMB in creative, but this should have survival use too. The issue with wires is that you can't click onto them, because they aren't a block. I think this shift+mmb onto Connectors/Relays thing could relieve some tedium during building. Probably should have a manual entry, too, though, otherwise I'll be the only user who knows about it :x

:D hooray i am helpful

I don't think there is a simple hook to do that. The function used in middle-click is getPickBlock but since it's not client only we can't just check for keyboard inputs there :/

rats. does the function provide the player and can you check if he's sneaking maybe?

...i didn't even think of that.
Yeah that's probably doable then! I'll take a look soon.