So some question, what cell service do you use? Can your phone accept emails? Are you running your front-end as your shell? Have you searched the net for any of the freeware programs that text your cellphone via command line? After all the above is done are you willing to present it to the SCREENLOCK author for inclusion as an option of the original program?
It's a great concept, but kinda unfulfilling. I can think of few places I'd leave my car open where I wasn't close by and my alarm has 1/4 mile range. Auto service is one, but the PC is usually disabled or removed (dealerships have a hard enough time diagnosing engine and tranny issues so I don't distract them with the tablet and joystick.) Valet parking is another, but again it easier to disable it than explain it. (actually, after reading another users post about his ar being driven over 100MPH at the dealership, I leave it on recording the GPS and shut off the screen and joystick.)
In my case, I probably could use it to notify me if the tablet was removed from the dock and then every five minutes after while its on it could send its GPS coordinates, but even then most snatch and grab car breakins are over in 60 seconds, and I can't imagine the cops believing me when I tell them someone broke into my car, stole my CarPC, and my tablet is at these coordinates - LETS GO GET THESE BASTARDS!
I'm sure there are a multitude of other uses for your car to text your phone (inside temp over 150F, outside temp below 0, alarm trigger, etc.) Keep me informed of your progress.