I'd wonder (and try to get to the bottom of) why your alarm was going of all the time rather than just by passing it. My (aftermarket) alarm kept going off once and it turned out to be the copper contacts on the boot connections were dirty and making the car think the boot was always open.
You'll not be insured either if they've disconnected the standard factory alarm, unless you've told your insurance of course.
I can't think of a live feed that goes off once your ignition is on though I'm afraid. The boot isn't supposed to be openable (sp?) when the ignition is on : it stops "oiks" from stealing the contents of your boot in traffic jams / high street etc. It can be annoying though.