Holding tank is an option but not conviient or low cost (Long run
cost). Here is a description.
http://www.laundry-alternative.com/holding_tank.html

If you went that route you could look into splitting some wastewater.
Laundry discharge to the yard for example.

Also, everything would need to be made low flow. I have no experience
with these systems but it sure beats making you own waste processing
station! Any chance public sewers in the future? Maybe all you need is
a stopgap.