How can I thaw a frozen washing machine pipe limited access semi-exterior wall?
I literally just got done dealing with some pipe that burst after it froze not 10 minutes ago. My first suggestion would be, after it thaws, make absolutely sure it didn't rupture anywhere. Even a tiny trickle can turn into black mold problems down the road. 

If you're using the type we used to use up in Canada when we lived there that's made to allow the expansion/contraction of freezing and you're less worried about rupture, what we used to do to avoid having to open up the wall was cut a small area you can easily place one of those cheap small vents into in your wall so it still looks nice. Then we'd either throw a space heater at it. Or we would go get those hand warmer heat packs that cost about a dollar a piece that when you rub them together they activate and produce good heat for multiple hours and lay them over the pipe to speed it up (ALSO to prevent them from freezing on the really cold nights.) Something like these: