Okay - so how to do this - well one way.
The key item you will need is a side inlet float controlled switch for a toilet cistern - this will shut the water off when the tank is full - all you then need is to connect this to the gutter or your neighbours down pipe depends on how keen they are...
Not heard of warranties being voided or extreme foam formation, but some of the statement is true in that you will get more foaming with softnened water than with hard water. Detergents contain chemicals that combine to remove the dirt from the water - aka soap. Scale also also reacts with...
I know this is a very old post - but it still links up on some search engines and this information is just wrong - washing clothes in softened water means you can use less detergent as the detergents are not having to compete with the water hardness. There is no chemistry for dirt and water...