Staffing is limited so they can't accommodate visits for anybody that asks. I think if they know they are letting someone come in that is helping share the information with a large group of people they are more likely to approve your request and if you've helped them in some manner such as donating accessions, etc. it is helpful. I know of someone who went without any approval and toured around on his own and they weren't too happy about that but were still pretty willing to help out.
During different fruit tasting events they've had they have sometimes been out in the field and sometimes at the buildings, inside our outside. Some irrigation equipment was once damaged by a vehicle near the pomegranates and that sort of thing certainly reduces the chances of letting visitors out in the field. Those sort of fruit tastings get announced through some of the California Rare Fruit Grower chapters but no other public notice that I'm aware of as they get crowded enough as it is.