I like the project - but I'm biased, since I'm more or less in the same boat
I've been using various Palm Pilot models for this purpose for years - driving a relay via serial port: SEE HERE.
I intend to replace the Palm Pilot with a Raspberry Pi soon, and I find your solution would fit rather nicely
However, given that RasPi is driven by Linux, I'll have to do something similar from scratch, most likely using Python and some kind of web interface for setting the process(es) up... Most likely add a small LCD screen for RasPi (cheap car rear camera display from ebay) for visual feedback. It will be another winter project, I guess