BG - the restrooms are just slightly north (well, west) of SART on the beach bike path.

You can see the little hut here: (satellite view)

http://maps.google.com/?ll=33.63298,...04128&t=h&z=19

I hope you're riding up from Long beach - PCH is a fun, fast ride to Costa Mesa, not sure about the return trip since I've never actually done it. Ride on the street, not the beach bike path. (unless you want to go really slow and dodge roller bladers)