Found it: it's actually in one of Jefferson's Pavilions (Pav II) at the University. Here's an image of the relief.

This discussion on Pavilion II's design says that the ornamentation follows a Roman model:

The drawing that Jefferson prepared for Pavilion II carried the notation that the order was based on the Ionic of the Temple of the Fortuna Virilis in Rome, which was illustrated in plates in The Architecture of A. Palladio; in Four Books. One of the distinguishing features of this order is that the volutes of the capitals at the outer corners of the columns are positioned diagonally at the ends of the portico. The Temple of Fortuna Virilis has a frieze of ox skulls, putti, ribbons, and garlands festooned with fruit motifs, which were also used in Pavilion II.