This is a fun topic... let's get into it! A couple of ways to skin this cat...
The way I would do it, which is arguably the most expensive but automated way, is to implement an electronic regulator. An analog signal into it from your controller would determine the CO2 pressure output. With this type of system, you can set the pressure via the controller, and dynamically adjust it as you see fit. For example, you could have your keg(s) pressurized according to a profile, such as 25 psi for 2 days, then 15 psi for 2, then 12 for 5, then 11. This would be an open loop system as you are setting the pressure but relying on the electronic regulator to guarantee the pressure. SMC Pneumatics makes a model ITV which could do the job.
The way mentioned above may also work (though I am not sure how much the pressure will drop once the feed is closed). This could be done with a solenoid and analog pressure sensor. The way this would work would be using a solenoid valve to open a valve connected to a primary regulator (set at, say 25 psi). The valve opens, letting CO2 into the keg, then closes. The pressure in the keg is then monitored. As the CO2 is absorbed, the pressure will drop to an equilibrium pressure, which the pressure sensor will read. If that pressure is not the intended final pressure, the cycle is repeated. The problem with this method is that after the liquid is carbed (and stops absorbing CO2), any drop in volume will result in a drop in pressure. If your control system is too slow, it could allow more pressure than desired into the keg (since the feed is ~25 psi), which will cause not only the liquid to carb higher than desired, but increase the serving pressure. Therefore, to make this work, the pressure going into the keg must go through a very small orifice to throttle the rate. This will give the control system time to adjust/sense/communicate/read pressure changes. You are effectively making an electronic regulator doing this. Since low flow is acceptable, I would use very small, piloted 2-way gas valves. Piloted means they use the incoming pressure to switch the valve rather than relying on ginormous solenoid/coils which require a lot of power to run and be switched. You would need flow orifices inline - I would shoot for a size which guarantees a long fill time - like on the order of 10 minutes. This would be a closed loop system as you are monitoring the pressure. SMC makes a model NVJ valve which is perfect in this application - manifolded, you can run lots of kegs easily enough. For pressure sensors, I would use a low-cost PC board mount unit like those from Honeywell if trying to really cut cost. SMC also makes analog pressure sensors.
Note: No real plug for SMC... I used to work for a distributor so I am familiar with their stuff - that's all.
