Since the Katella restrooms are closed you could try using the ones inside the Amtrak station ( the silver domed building). I've done it before, the staff doesn't seem to mind as long as you don't look like a homeless person (FWIW: I've used the train to take my bicycle somewhere, so bicycles are not unusual inside the station). I haven't been up to the Katella/stadium area in about 6 months.

I'll have a look down at the beach this weekend to see what's open. That first restroom building at the south end of the SART is a popular stop for bicycles, and there were quite a few folks taking a break at it last Saturday morning. Huntington State Beach was allowing persons to cycle/jog/walk along the bikepath, and surf in the ocean, but no stopping, but it all might have changed with Gov Newsom's directive earlier this week.