This thread has some info http://forum.linuxmce.org/index.php?topic=8024.0
Rain8 and rain sensor could be created as separate devices which could provide better information. So rain8 setup for controlling the irrigation and then something like the 1-wire rain gauge would monitor rain fall. This would then allow the system to monitor how much rain has fallen plus trigger events in LMCE as to whether irrigation should be turned on. Plus all the timed event functionality is already built.
Also you could use soil moisture sensors (also on 1-wire) to monitor moisture sensors in each irrigation section to control what is needing water depending on their requirements.
Regarding tap presuming you can get to your piping it is not too hard to do. Can get push together connectors which apparently is very easy to use (I'll hopefully be doing some over the next few weeks). Otherwise should be a fairly quick job for plumber.
1-wire devices I mentioned are currently not implemented but big thanks to Hari and Possy for doing the work they have on getting the controller working with temp sensors. It is a very cheap alternative to some of they others like KNX etc.
This is all off the top of my head so anyone feel free to smack me over the head where I'm going wrong