To answer you original questions:
– Is there a advantage in using either the 4 FET or single FET design?
OSBee operates latching valves and for that reason, it needs each zone to use a half H-bridge to be able to reverse the polarity of voltage. This is different from OS DC, which only operates non-latching valves, and hence a single FET is sufficient.
– Will the current firmware for the OSPi support the DC design?
It should be able to. The difference in AC vs. DC is mainly in the circuit design and not so much in firmware. In fact, the primary difference in the firmware code is quite small — because OS DC needs to boost the voltage to energize the valve, the apply_all_station_bits function has additional code to turn on the boost converter in order to boost the voltage. If you don’t need the boosting stage (say, because your valves are all 12VDC and your input voltage is already 12VDC), then you don’t even need the boosting stage. In that sense the AC and DC has really no difference in firmware code.
– If so is there a preference for which PINs on the Raspberry Pi I should connect BST_PWR and BST_E?
BST_PWR basically controls the input voltage into the boost converter. When it’s enabled, the boost converter starts and bumps the input voltage to about 22VDC into the 2200uF capacitor. BST_E controls the output path — it should be disabled when BST_PWR is turned on, because having the output path enabled may affect the boost converter and make it no able to successfully boost the voltage. Once the capacitor is charged, turn BST_PWR off, and BST_E on, that way, the boosted voltage will be present on the output path. In the firmware code (apply_all_stations_bits function) this is pretty clearly shown there. You can use any available pins for BST_PWR and BST_E — these are just general purpose output pins, nothing special.