Washing machine outlet hoses need to be brought up to just below the
top
of the washing machine anyway - they don't just drain along the
floor.
These outlets are pumped, so as long as the height difference is less

than (say) 24in you should be fine.

That's a good thought but my macine will be about 20 feet from the
nearest point I can connect to the sewer - doesn't this imply that I
will constantly have grey water lying in this pipe work ?
How should I protect against flowback into the outhouse?

* and thanks for your helpful replies ; )