I was wondering if it's possible to pickup the 2.45ghz signals from the waveguide or cavity of a microwave oven, perhaps using a simple dipole antenna at ~4.5inch lenth to match? Any ideas?
Yes, but I very seriously advise you not to!
Power in the cavity (which is inside the magnetron so you can't reach it) and the waveguide is VERY dangerous and could even melt your dipole! Consider that in a microwave oven the power in the waveguide is spread throughout the cooking area and still cooks food.