@crossmax, I think what you are looking to do is use a single raspberry pi to run the OSPi firmware and to also control a latching solenoid? This is different to my situation. I do not have OS firmware controlling GPIO pins to drive latching valves. The firmware is not geared up to do this out-of-the-box as you mention above.
In my case. I already had an OSPi controller running all of my stations, programs and regular valves out in the garden. My goal was to find a way to add an indoor latching value into the system without having to run wires into the house. So I wrote a bespoke program to run on an ESP8266 that simply “listens” for on/off commands from the garden OSPi and in response opens/closes the latching valve. So my solution has OS firmware running on the garden RaspPi and a simple gpio routine running on the ESP not a combined unit.
You can check out the this post where I set out the solution. I’m happy to post the code if useful and talk through the hardware but it was Arduino based for an ESP8266. So it would not be a simple copy and paste onto a Raspberry Pi.
If you really want everything in the one unit then I think you would need to either extend the OS firmware to support latching valves or alternatively create a “man-in-the-middle” programme that monitors the OS firmware status and then appropriately configure the H-Bridge hardware. You should also check out the new OSBee 2.0 unit as an option
Happy to talk through.