That was the first design but the bugger kept pecking holes in the tubes and I had enough bloody leaks already!
Version two was a pressure relief valve built into a replacement cap, the idea being that you screw on the airtight cap which presses in the valve, and there is a blowoff valve built into the cap which operates above whatever pressure you wanted. It was more of a concept than a detailed design though I did a drawing based on a technical drawing of an existing Leafield valve, but it should have been perfectly possible to make.
I suppose they wouldn't want to sell just one product for each chamber when all the mugs that want them currently buy two separate ones ... my idea is the Wash n Go 2 in 1 of rib tubes, why take 2 bottles into the shower