For going out you could use a PIR sensor as you assume once in it's your dog that wants to go out. You could use one for coming in also but you'd not have any verification it's your dog. It would be the same result as a magnet though. An advantage to this approach is cost. PIRs are cheap, don't use much power and most have an adjustable range feature.

If you didn't mind something more interactive you could weigh your dog and input that to your PIC or have a learn mode. Either might be painful as the dog might not step on entire pad, might drink alot of water or get caught in the rain. Any would throw off the saved weight.

I still wonder if RFID might be your best least complicated for the dog solution. Though usually as soon as I say it someone else says I'm wrong. Good Luck.