My app operates across arbitrary servers, and if you happen to have not only JRiver installed, you will find that the other ones dont send default icons (and certainly not the JRiver one). That makes the browsing experience inconsistent even from an aesthetic POV.
I guess it depends whether people are generally going to have one server and many clients, many servers and one client, or many servers and many clients! You're not going to get a consistent browsing experience unless every single client and every single server is configurable. You are working on a many servers to one client principle. I have one server and many clients, and my browsing experience
is consistent, with regard to icons, due to the fact that the server is sending the artwork, with those clients that respect artwork sent for container-level items anyway. I also don't like those clients that have their own ideas of the way my collection should be sorted!
By the way, I'm pretty sure that Synology's DLNA server has a certain amount of default icons, and Meedio (when I used it) did as well, but don't hold me to that!