I spent some time in OpenSCAD and have a working, parametric design for the shelf pin.  The design looks good in the viewer and has parametric hooks in place for the pin size and the shelf thickness.  Other measurements (like the length of the horizontal arm) are currently set to a constant because I don't envision them needing to change.  Since they are set by a constant, however, I can change my mind on that without having to chase hardwired constants through the code.

I made the bottom support on the pin a little more rounded, for a cleaner look.

As it stands today, the OpenSCAD code works in OpenSCAD, but is not yet working correctly in Thingiverse Customizer - that's the next step.