I love that video - we have a similar problem with cats here (and foxes / stray dogs / chavs )
I'll be doing a similar thing myself at some point as the little buggers keep pooping and digging up the bit of turn next to my front door.
I wouldn't go with the beam break to be honest - as soon as it rains / gets dirty / bugs walk over it / foggy / condensation etc you'll have issues. Maybe mask off parts of the PIR to get the best effect ? I've seen this done before to give a very narrow field of vision.
I'm personally tempted to make some kind of ultrasonic 3d range finding device and map out what is in the garden then use some kind of targeting system to follow the little buggers around squirting them with soapy water or similar