The midi relay board should be all that you need. Make sure you order this version:
MIDI-RLY08 - 8 relay, 0 dimmer
It consists of 8 conventional relay contacts, each capable of 250 volts at 8 amps. You have five swell shutters, so you will have three spare contacts. The magnet on the pneumatic valve will be in the range of 150 - 175 ohms, depending on the manufacturer, so the current draw will be less than 100 milliamps at 12 volts.
I have the Paramount 341 installed. By right clicking on the main swell expression pedal on the virtual console, and selecting "adjust midi/trigger settings manually for expression pedal, I confirmed that the input to Hauptwerk is classified as Control Change, with a range of 0 to 127. By looking at the bottom of window, I could see that the actual range of values determined by auto-detecting that swell pedal is 17 to 67.
To control the relay board, you need to select the primary output tab, choose the output type as Midi control changes. Then select an output port from your midi interface, and an output channel corresponding to the midi input channel of the relay controller.
To configure the relay board, use the utility supplied by the manufacturer, Set the midi channel, and check the "CC enable" box. You can then set the initial values you have chosen to set the trip point for each swell shade motor. For example, if you wanted five steps between my actual values of 17 and 67, you could set relay 1 to close at CC greater than 19, relay 2 at CC > 30, relay 3 at CC> 40, relay 4 at CC > 50, and relay 5 at CC > 60 Once you get the virtual pipes and real pipes working in tandem, you can tweak the CC trip values to match the loudness levels more exactly.
Another subtle issue to monitor is the likely difference in latency between the virtual and real pipes. Johan Liljencrants has written an excellent dissertation on the latency of electro-pneumatic and direct electric valves. You can see it at
http://www.fonema.se/valvetime/valvetime.htmlIf you find that the virtual and real pipe latencies are too different, you can tweak the Hauptwerk latency by changing the buffer size.
Ray