I had imagined a ball valve mechanically hooked up to a servo - afaik, ball valves take the same amount of force to operate, independent of pressure (within sane limits).
Thanks for your reply, though. I'm starting to think about doing something like this, despite all the warnings in the original post and all the comments that it's basically doomed to failure.
It is basically doomed to failure; but you'll learn a lot and be happy for the experience. The fun part is that it seems so simple on the surface if you're someone who doesn't know much about gardening. But without the benefit of experience in gardening, you make mistakes like the OP that no amount of technology can solve. Automatic watering won't help you if your soil isn't draining.
And the ironic part is that it's actually not hard to cobble together an effective automation setup using some analog electronics like a cheap light timer, an AC fan controller, and a $20 automatic watering gizmo from the hardware store. I had a hard time building a more effective solution using an RPi or Arduino.
Thanks for your reply, though. I'm starting to think about doing something like this, despite all the warnings in the original post and all the comments that it's basically doomed to failure.