Depends on your definition of cult wine.

If you count Shafer, then I have one bottle of Hillside Select 04.

If you want to visit Shafer, advanced booking is strongly recommended as they only allow a very small number of visitors per day, and you have to pay US$40 (if I remember correctly) for the tasting.  As visitor, you are allowed to buy 1 bottle of Hillside Select and the price is slightly cheaper than internet-order price (but this one is truly ex-chateau).

Depends on what you want.  Shafer is one of the very few that will let you visit their wineary, and the owner also come out and have a short chat with the group.  The typical "visit" of others are simply their cellar door where you taste their wine (mostly free) and then buy.

The best way is if you know of any winemaker event of a Napa producer, go their and tell the producer that you will be going to Napa and like to visit their vineyard.  If you are able to show them your interest, knowledge, and passion, you will be well treated.  Most likely they will let you join the "trade tours" i.e. tour for those who work in the wine trade.