This is certainly an interesting idea, something we haven't explored because we focus on Fine Art printing applications mainly. To mix a dilution of mixed inks in with a black you would end up with a colored black, probably more towards the brownish side, this would not produce good results while printing images (obviously). One other thing to consider is when the ink passes through the capping station on it's way to the waste tank it can pick up left over pigment inks that may have congealed or collected in the capping station, essentially making it a recipe for producing clogged channels in your print head. You may consider this ink a used "oil" of sorts in this aspect. For that reason I wouldn't recommend re-use of the waste ink in the printer itself. Rather you could consider using the waste ink as artist ink for painting.