How much waste are you guys making that you guys are afraid of somehow damaging your septic system? There's nothing in there that would harm the tank/fields, and the water volume can't be that much. How much water do you generate in a shower? Say a 15min shower at 1.5gpm, that's 22.5g of grey water. Assuming you have multiple people showering per day, lets say you toss in 135gpd for showering alone. Add in laundry, dishwasher, and toilet flushes and it's even higher. I don't see why putting in say 80g of waste (assume 20g wc with 80g waste) once or twice a week would somehow break your septic system.
Not to say that re-using the waste for watering plants and ponds is a bad idea, I'm all for it, I just think concern of ruining a septic system with some nominal amount of waste water is unrealistic. Newer septic designs where I live are for 110gpd per bedroom. Older homes I believe were for more, as they pre-dated low flow fixtures.