Johndoes method is the best imho since you can't change definitions on the fly. When the zone trips, you manually sound the siren, etc.
My issue is I think you will get alot of false alarms. Unless you have a completely fence in area and no wildlife or anything, you will get nuisance trips. I would either use a curtain type sensor - Optex makes a good one like
this or a photobeam like
this, or a wired screen. Believe me, the cost will save your insanity the first time you get startled out of bed in the middle of the night because of a nuisance trip.
I would feel more comfortable using these to trigger an alarm outside instead of a regular PIR. Then I would set them up as a timer like johndoe described, except I would not wait the hour, I would just use these and have a rule that if they were tripped maybe I would flash a light or chirp a siren as a warning, then if it tripped again within like 10 seconds, sound the alarm, etc.
Edit: Ok, I missed the point entirely, you are wanting to use you internal PIR for this. Well, anyway and outdoor detector like I mentioned would still work.