Well, you don't need to be endorsed, but you need to somehow secure a pass, and over the last few years, those have gotten harder and harder to get. Best way to score a pass, is have a connection with a company that's going to be at NAMM. Companies that have booths there, get a certain allotment of passes.
Not sure about a "public" day but NAMM offered members several 1-day, Sunday only passes for $10. Sponsor said "they change the rules every year". I guess it's anyone's guess what will happen this year.
Keep in mind that NARAS has only a limited supply of badges available, membership does not guarantee admission to NAMM. Also, different levels of membership have different benefits. As a voting member, I was offered a badge each year, but I don't know if all members get that opportunity.