I am testing undilluted "Original Bc" Bacteriostatic treatment on a penny to see if any corrosion occurs. It is designed for humidifiers so I imagine it has negligable long term effects on plastic. Its clear as water. I will use that in my no Water Wetter havin WC system if it works out.
*EDIT added link to product..I bought it at Lowes
http://rpsproducts.bizhosting.com/3bt.html