Any elements that can be connected to the cavity under a stud?
If I'm understanding you correctly, you're talking about the molding cavity directly underneath the stud (that actually goes up into the stud), and not the main tube cavities under plates/bricks (that are actually offset from the studs).

If that's the case, no, I'm not aware of any element that "intentionally" fits that space.

First off, I don't think that space is made with a high tolerance.  Looking at some of my bricks, the dimensions of that cavity are not nearly as uniform as the other features of the brick.

Secondly, I don't know how many System bricks have components that are not integer multiples of LUs.

You might get lucky and get a minifigure piece that has a detail that is 1.5 LUs, but you'd have to try a lot of them.