Wanted to give a small update on that particular issue. If albeit a late one.
Just in case someone else reads through this thread at some point and encounters the same issue with any other kind of short shifter.
As it turned out, the vibrations came from the uniball joint of the shifter itself, just as I suspected. By design, uniball joints always have a little bit of play in them and that created the rattle noise.
But the harder bushings definitely were a major factor here, as well.
I dribbled a bit of chain/machine oil into the base and after two or three applications the rattling completely stopped! :D
You know, the kind that's very thin and oily at the start but quickly gets more sticky and viscous as it reacts with the air?
That stuff also makes great chain lube for motorcycles, btw. ;)