As far as I can tell, the rain sensor is basically a simple switch with felt or foam attached to it. When the felt/foam gets wet the switch is activated. Depending on how your system is configured the switch can be set to be normally open or normally closed. Both wired and wireless units are available and can be had for as little as $15 (wired) or $50 wireless. I have a $15 wired unit and it seems to work well enough. I imagine that you can simulate rain sensor operation simply by shorting the the Rain Sense pins on the OpenSprinkler board. Please verify this last statement as I would hate for you to damage your board if I’m wrong.
On the topic of logging, while I would love to see rain sensor data logged, I can now see the limitations of the current logging system and I wonder if time would be better spent looking at creating more flexible logging system. I believe all machines running a web server (including my router) will be capable of using some kind of database package and perhaps taking the output of the OS device settings command and dumping that into a line in a database file could be the answer. If done correctly I imagine gathering statistics and even generating graphs would be pretty straight forward as that’s a fairly common use of databases. I think how you choose to move on logging and rain sensor data really depends on what your vision is for your webapp. It’s easy for me to come up with ideas unfortunately I can’t offer much coding support.