I'm not terribly familiar with washing machine outlet boxes, but it looks like a lot of them have brass/copper hardware, which wouldn't be good for the tank. I don't see why you couldn't remove the included hardware, and replace it with aquarium safe stuff.
or even just drill a couple holes through the wall and run tubing through it. -it might be better to not have any joints/unions/whatever inside the wall, since you wouldn't be able to detect a leak, and that could lead to water damage, rot, mold, etc.
(assuming the typical american home construction with drywall and 2x4's)