Your cheap solenoid thing is something I have also pondered with, as I need something in the 25mm diameter range, and I refuse to pay the $120-150NZ they want.
For your application, how about a simple low pressure ballcock.
I know they work at zero pressure as I have used them to feed a trough from a water tank sitting on the ground.
A small elevation will help, and each metre of elevation should give 1.4 psi (see http://www.engineeringtoolbox.com/pump-head-pressure-d_663.html
The pressure is determined by the level of the water surface, so a tank with 2m of water in it has about 2.8 psi at the bottom.
If you have a limited water supply, you could use a seperate tank, which is only filled when the outlet ballcock is in the closed position.
A simple level detection on the side of the small tank (20 litres) would allow for registering the amount of liquid used over the period.
The other option to use a small tank, which is filled based on the amount required, and use a low flow discharge from it to the garden plants.
As long as the water flows out, it doesn't matter if it takes several hours to empty.
I have also seen the outlet for such "non eassentials" higher up on the water tank so that there is always water for the household.