Hey Chris, I sympathize, I’ve missed a few events by forgetting to register on time, if you’re not registered by 1PM the day registration opens odds are slim to get in, they make a good atmosphere that keeps people coming back.
Luckily there’s lots of opportunities to autocross, I’ve enjoyed events by the Golden Gate Lotus Club, Loma Prieta Porsche Club, and SCCA San Francisco Region at Marina. Looks like the Porsche club event at Marina on Sunday isn’t totally filled up, maybe they can fulfill your need for speed?
If it’s not too far there’s also American Auto-X Series at Crows Landing this weekend, no pre-registering, show up hand them the cash and take your runs, SCCA also puts on events out there and it’s a great site. Hopefully one popular sell out club hasn’t scared you off the hobby.