The drain pipe for my washing machine is not in the floor, it is on the lower portion of the wall behind the washer. How can I set up the overflow pan to drain water with no hole in the floor?

You must have a trap for the overflow pan so you probably couldn't hook the drain pan up anyway unless the trap is lower than the floor level. Of course then the washer drainage would back up into the overflow pan. NOT what you want. Usually the pan is drained below the floor through a separate trap.

Otherwise just put a water sensing alarm in the pan. It will scare the daylights out of you when it goes off because you will have forgotten it is there by the time you need it. Or else the batteries will be dead, which is why I will only use the plug in ones. We have several pans that have no drain. Usually (not always :( ) a washer leak starts slowly and you will see a bit of water in the pan in time to shut things down before a disaster.

When we install washer or a water heater pan we pipe the drains outside with 3/4" PVC. They can not be drained into the washer drain. We build a small trap at the terminal end of the drains to keep out bugs and snall critters. Good luck, Tom

We don't know the details about your home but I don't see much of a need for a trap on this pan. Is this the first floor and is there a basement or crawl below. If you were to drill a 1" hole downward behind the washer what material would you be drilling and where would the drill bit pop through?