You could always try repeatedly dividing by pi, keeping track of how many times you've done it, and when you finally get a rational value, use the graphscreen trick to make that value into a string. From there, it's as simple as concatenating a "pi^" and the number to the end of the string.
EDIT: From my experience, making the output prettier is almost never worth it in BASIC. You're better off just displaying the value and making the users figure out how many pis are in it, the lazy sots.
