The OP said he lives in northern CA, their climates can be anything BUT mild, depending on where he lives. Mountainous areas are as cold as many northern states. I live in the central coast region of CA, about 10 miles n of San Luis Obispo, and I got 1/2" of snow/sleet Friday. The ski mountains near Fresno, have a 15ft base of snow. everyone thinks the weather in CA is all beach weather.
As for the freeze protection, I was here for many solar installs in the 80's and most I saw used the "drain back" or drain down systems. The systems had a vacuum break on top and the collectors would drain completely when temps got below 40 deg.
Does the system in question, have a line from one of the valves that leads to a drain? If so that is what you have, most likely. I cannot imagine a solar system that "doesn't need" the circ pump, unless it's a batch system, with the tank on the roof.
The gas valve system is no big deal to remove and repair. I got my associates deg (many years ago) in heating/ Airconditioning and refirgeration, and then worked as a service repairman in the field for the next 5years, before getting hired on as an air traffic controller, in 1982.
You should be able to remove the valve/burner assembly as a whole unit. It's easy to see the pilot opening and look into it with a flashlight. To clean it, it's best to disassemble it from the aluminum or steel (they used either one) pilot line, and then remove the orifice. Use wrenches to back up the nuts when loosening, to prevent from twisting the pilot line. The orifice may or may not be threaded in. When cleaning, use a soft bristle brush, like a tooth brush, NEVER a wire brush. Also use compressed air. never insert anything into the opening in the orifice. If it gets enlarged, it can overheat the thermocouple or worse.
Reassemble, and relight.
If all that sounds like too much, then as mentioned above, call a pro. in the 80's all systems, I ever saw, the gas hot water tank was really an independent system, in that it simply would only turn on the gas burner, when incoming water temp was BELOW the min setting for the gas tank valves design. In other words, if the gas valve was designed to open at 100 deg, and turn off at 130, the gas valve would not activate, ever, as long as incoming water never dropped below 100 deg. The quote you gave above, sounds just the opposite, of normal design, and I suspect you either misread or misquoted.
If the storage solar water tank temp is 110, then I doubt your gas hot water heater will activate, even once the pilot is relit.
As mentioned before, after 25 years, it might be time for a system upgrade/overhaul.