In our bathroom, we've got half a dozen recessed halogen lights in the ceiling, all operated from a single pull switch.  Would it be possible to also have a pir sensor in the ceiling that just switches on one of them?

I realise it's a weird one, and I think the answer is probably "no"

I had a pir fitted to the landing lights a few years ago to stop the kids automatically turning the light on (but never off) every time they walked past.  

This time it's for the cat's benefit!  Trying to get her to use a litter tray in the bathroom, as its a bit nicer than having it anywhere else in the house.  She uses it fine during the day, but at night she won't go in the bathroom unless the light is on (particularly weird as our bathroom hasn't got a window, so it's still dark in there during the day). Wanting to only switch one light in the bathroom to cut down on the power use.

How about switching them to LED bulbs while you are at it. 50W becomes 5W. Look for bulbs that produce >400 Lumens. You probably won't find them in the DIY sheds but lots on ebay or Amazon.