Really?
2002. BLUE DRESS UNIFORMS (See Appendix figure 7-32)
1. The blue dress “A” uniform may be worn for parades, ceremonies and formal
or semiformal social functions, for NCOs and below throughout the year and
for officers and SNCOs as appropriate to the season or those occasions
requiring uniformity with NCOs and below. The blue dress “A” uniform will be
worn for the following official military/social occasions:
a. Parades, ceremonies, reviews, solemnities, and entertainments when
the commander/senior officer present desires to pay special honors to the
occasion.
b. Official visits of, or to, United States civil officials, officers of
the United States Armed Forces, and officials/officers of foreign
governments, according to chapter 12, U.S. Navy Regulations.
c. Receptions given by, or in honor of, officials/officers listed in
chapter 12, U.S. Navy Regulations.
d. At daytime formal or semiformal occasions.
https://www.marines.mil/portals/1/Publications/MCO%201020.34H%20v2.pdf?ver=2018-06-26-094038-137
No mention of authorizing of a civilian cap and gown to be worn over it. There is an over-coat that is authorized for inclement weather and outdoor ceremonies.
Better for the Marines to wear their Blues in and out but shuck down to their skivies and put on the gown to receive their diplomas. Disguising ones uniform by putting a tablecloth on over it can get you suspected as a spy and getting one summarily shot. Don’t disguise the uni.