The “low-tech” method that is not software controllable is to put a flow restrictor on each zone that will give the flow level you want. You can use fixed flow adapters or a manual ball valve that you adjust to get the flow you want.
Programmatic control of flow would require a flow meter in the line hooked up to one of the sensor connections, and use of an electrically controlled variable flow valve. Electric variable flow valves are typically only used in industrial settings and are consequently quite expensive (around $500 or more). I’m pretty sure that the software doesn’t yet support control of a variable flow valve, but it should be possible if you can find a suitable valve.
I actually found the opensprinkler project because I was doing research trying to build my own system that had software control of both pressure and flow. At the moment I’m trying to dig far enough into the hardware and software designs to figure out if OS can be adapted to do what I want easier than building my own system.