Hi all,
I thought I'd post back and confirm that I have both OpenSprinkler and HG working on the same pi, as Dani and bkenobi said it would.
I just installed HG as per the instructions on the downloads page on top of the card image for OpenSprinkler.
I only had two minor issues I had to deal with. First was getting the download onto the card. I couldn't get FTP to work, so I ended up using the UNIX "scp" command, which worked fine.
Then I had some errors return during the install. The 'downloads' page of HG suggested installing the 'en_EN' locale. I couldn't find that, but I did install 'en_US' to supplement the already configured 'en_GB'. When I re-installed the errors disappeared and the system is now working.
Cheers,
Tony.