The Officials Code of Ethics appears as Appendix G to the NF Rule Book. It does not directly address this issue. It does provide that “Officials shall be punctual and professional in the fulfillment of all contractual obligations,” and “Officials shall uphold the honor and dignity of the profession in all interaction with student-athletes, coaches, athletic directors, school administrators, colleagues, and the public.” But nothing specific to this point.
I confess, I got a t-shirt from Penn once, and they also gave me the game ball when I ended my on-field career there in 2017.