First of all thank you for discourse! It is amazing. I just finished my RPM packages and helped integrating it with pixls.us. Together with Pat I was looking into the multisite option.
pixls.us currently uses the rss parsing + embedding support shown in @eviltrout 's blog post. In the current design only one site can embed back. One option to fix this would be to have a mapping category->site and allow embedding based on that mapping. In the case of Pat, he thought about sharing his instance with the libre graphics community. For me personally I would love to have discourse as common commenting platform over all opensuse.org sites. The mozilla team was discussing similar things here.
There is already a feature request topic for this.
We definitely want to make it so that RSS feed embedding can be done per category rather than only once per site.
I realize this is old, but couldn’t find another topic that was quite as on-point (I’m the Pat referenced in the OP).
We are starting to use pixls.us as a commenting platform for other libre graphics projects, and this problem is rearing its head again. Is there no way to specify more than one RSS feed to poll for posts still?
I love that I can at least specify multi-site embeds, but pointing to an RSS feed would be preferable to attempting to parse the html of the pages.