In principle, JRiver is already that audiophile grade middleware via ASIO to DLNA. It exposes an ASIO inteface that roon recognises. Many on this site came to roon from JRiver and still maintain licences. With a BubbleUPnP server it provides access to chromecast as well.
The only thing is I cannot get it to work. JRiver integration seems to have been a hotish topic a few years back in the early days of roon. I don’t know if it is me doing something stupid or it’s just not supported anymore. I’ve raised a support ticket here: