Definitely take care with the products you use in your washer but it's such a great idea!
All my plumbing is in the slab so I can't do anything cool like that (couldn't afford to have it done that is). Please let us know how it works out for you!
PS, make sure you use a lint screen for the output line. You just tie wrap it around the hose as it goes into the barrel and clean it out periodically. It'll save you plugged hoses and much trouble later on! (presuming you are going to use some kind of hose nozzle or drip irrigation, etc.)
Here's an example so you have an idea what I'm talking about:
"Sweetfilter Washing Machine Filter Lint Trap Can Double The Life of Septic Tank Systems"
http://www.amazon.com/gp/product/B0006OIHAU/002-6520871-9215218?v=glance&n=284507But you don't have to buy one, you can make it yourself out of extra stuff you have around the home. Just make sure it's at least a few inches long as it will get clogged with lint too quickly if it's too small.