A brief test today proved that both the spring and E-clip samples provided by Century Spring will not work for me; the spring (although tiny) provides too much resistance, and the E-clip (the smallest size they supply) is too big. I think these tiny solenoids aren't going to work.
However, another contender has appeared. These solenoid valves might work for me, although they might have a problem with throughput if the connections are too small. I'll see what I can do.