No, I still haven't figured it out. I was wondering if there's something in the new firmware update that will allow it.
For others reading this: the point is to do this at the router-level, NOT through editing an /etc/hosts file, so that all device on my network (including multiple iPhones and iPads which don't have an option for editing /etc/hosts) can all block the websites I want to block all at the same time. With other routers I would just add a domain and have it route to 0.0.0.0 like one would do with an /etc/hosts file.