Is this official "no drinking in letters" thing some what recent for NPCs?
When I was an undergrad at Kentucky last century (late 1970s), one of the "top" fraternities had an All Greek Beer Blast. To get in, you were suppose to wear your letters. The reason being that only Greeks would be wearing letters. And because they were a "top" chapter, you bet every sorority was out in force, in their letters, with an ice cold beer in hand. Unless they were doing shots of bourbon!
For what it is worth, this was one of the best All Greek parties. It was normally held on a Friday (often before a home football game) and had live bands and hundreds and hundreds and hundreds of kegs. House Mothers had a reserved area where they were given priority treatment. The Dean of Students, Greek Life Advisors for NPC, NPHC and IFC, as well as chapter advisors etc. would all be in attendance. It was so popular that around my senior year, so many Greeks from other campuses would crash it that they started to require - in addition to letters - a current UK ID to get in.
Also "way back when", there were certain nights when a chapter would wear their letters out to the bars for what ever reason.
And this was not just restricted to UK. I experienced it at other Kentucky, SEC and Big XII campuses. Also at Indiana and Maryland while away attending football games. Frankly, there were lots of sorority women wearing their letters and tailgating (i.e. drinking).