Andy, it can be done. I've gone through the same process on a small repairs on my boats. You're right, Sanger white is slightly off white. I added a drop or two of black pigment to the white, until I got it right.
In San Diego, you should have lots of fiberglass repair places where you can obtain the goods locally.
If you can't find anything, this is where I get my supplies, they are just a couple miles from my house, maybe you can mail order from them...
http://www.crystaliner.com/