Thanks for all of your varied, informative, and sometimes weird and offensive answers.
Just to be clear: the reason I was asking in the first place was out of a desire to 'play by the rules'. Since the race organizer included no detail beyond "self-supported" and "no outside support", it was my assumption that there was some kind of general consensus in the gravel-racing community regarding what specifics these terms entailed. It's now apparent to me that there is no such consensus and that clarification needs to be sought from the race organizer (which I have now done: his ruling was 'no - not allowed').
No thanks and no appreciation to those whose answers implicitly or explicitly accused me of trying to gain an unfair advantage in the competition, or who assumed that I was looking to garner support for knowingly cheating. I took those comments personally. Also, the answers that suggested I simply carry more water were not offensive to me, but they were completely useless: I realize that it's possible to carry more water....it's always possible to carry more water.
Thank you to those others who gave respectful, useful advice, or shared your experiences.
I didn't want to add so much background detail to my original post, but now: I did this same race once before (several years ago, it was the race's inaugural event, and also my first-ever 100 race). Not only are there zero potable water sources along the route, but when I arrived at the one-and-only water/feed station around mile 65, they had just run out of water (there is no spigot there, they have to shuttle coolers of water up to the checkpoint by vehicle). Some 'racers' had to wait an extra 20 minutes to refill water. In the end, I wasn't upset about the logistics problems: it was an otherwise very well planned race, in a beautiful area - extremely remote, with good company and good will shared between all, and for a good cause. Plus it was their inaugural event, so I would expect some kinks to work out. Nonetheless, I have some hydration anxieties now. The reason I didn't share all of this background in my original post is because I realize that it has no bearing on whether or not stashing water along a course is common/accepted/kosher under races with 'self-supported' rules.