Generally, K, we reserve art requests for products that are likely to make it to print. Supplements, for the most part anyway, are not intended to be print products. What art I have used in my own supplements was mostly borrowed from other in-print items (such as the Gnome art from the core rules also appearing in the Gnome supplement).
Artists, like cats, may do as they wish; if an artist provides art for a supplement, we graciously accept it. But I generally don't ask them to do work on things that won't be printed.