Step 1: Designing the Charm
I started in Illustrator and found a font that had the profile I was looking for: Stencil Std - it is curly, with one π leg thicker than the other (giving space to write a name). The legs have to change, however: the should be bend inwards to grip the wall of the cup.
Starting with a 1" x 1" artboard, draw a π and scale it to fit the artboard. Next, turn the text into an outline with Text > Create Outlines.
To rearrange the legs (which sounds like something a mobster would do), you'll have to cut the path: Carefully draw a line across the corners of the leg that you want to cut, then use Object > Path > Divide Objects Below.
Finally, rotate each leg inwards until it creates a pinch at the bottom and leaves a gap at the top, for the cup rim. The only way I found this to get exactly right is to test out each change with a real charm until it works for the cups you have.
Step 2: Make the Charms
Instead I settled for a handful of 3D printed charms and decided to do the rest in laser-cut acrylic and wood using the vector image from the last step.
If you make sometime like this, experiment with sizes, shapes, and materials - I'd love to see what you come up with!