Simon,
I can see a way if I am right. The guru's will correct me if I am wrong.
Ok, fill your tank with tap water and using a shurflow pump, pump the water from your tank into a 22ltr DI vessel. Then from the vessel connect to the brush head. You may consider 2 x DI vessels to ensure your TDS is at 000
Unless the heat has got to me or I really am stupid I can't see how that won't work. The water is held in your tank, its pumped through the DI vessell and out through your brush head.
I use the exact same system only the water in my tank is already pure and the DI vessel is a small 12.5kg just to polish that 100%.
All I see your doing is increasing the size of the DI vessell to accomodate the water TDS levels and reduce to 0?
So, unless I have missed something, that will work??
Best wishes,
Trev