This has been discussed before , cant remeber where exactly, Most alarm systems do offer various outputs designed exactly for that purpose,
Some are hefty outputs i.e can drive a few hundered milliamps , whereas some are just for signlling 10-20ma (drive a LED or relay coil) some of theses outputs go high some go low, some stay on for a time output some stay on all the time, some are a quick pulse!!!, what ever your alarm offers make sure you interfcae correctly too it, external circuitry may be needed to handle such outputs
overall benefit is a system that starts booting when you disarm the system great! prehaps you could set the sysetm to turn off exactly 5 mins after the alarm has been set! that way itll save gas station annoyances!!
Hope this helps!