I will assign this to Development as a wishlist suggestion, but at present we can only set affiliate cookies that expire in 6 months, and even then there's always a chance the buyer could clear their browser's cookies, use a different browser or computer, etc. before the 6 months are up. Unless the buyer goes thru the affiliate's link for every reorder, at present we really don't have any way to support ongoing commission "residuals" like you propose.