heads are easy especially if your halfway there and don't have to pull intake drain coolant etc... You really cant screw it up if you follow the torque sequence. Try the magnet and see but I'll bet it grabs the washer in the wrong orientation to get it through the valve
It can be done man. I've pulled a small bolt out with a magnet. I had to put wire in the spark plug hole to hold the valve. Used a valve spring tool to compress the spring to remove the valve keeper. Wrapped a rubber band and string around the valve stem to keep it from falling in the cylinder all the way. Pulled the wire out ( praying for my life ). Lowered the valve down. Used a flexible magnet through the intake port and fished that baby out. It was like that game " Surgery " only I was the one having fits every time the magnet stuck to the wrong thing. After what seemed like an hour it was out and the valve was going back together.
If you go this route you have to make sure the valve doesn't fall all the way in the cylinder or you will be pulling the head. Old clothing line string works best...but some electrical wire will also work. It's a pain getting it to ball up in a way to hold the valve but isn't impossible through the spark plug hole.