I was having trouble using kunenadiscuss because although the forum is shared, the content the plugin references live in individual tables. The problem is that when viewing a post created by kunenadiscuss in a site other than the one that created it, it will show the article from that site with the intended article's id.

I hacked up two files in Kunena and kunenadiscuss, and now it works pretty well. There are lots of other parts that would be nice to integrate better-- the parameters don't make much sense when there are different content categories on each site-- but this will do for me for now.

I'm posting the files here in case anyone else can use them or improve them. (I couldn't figure out GitHub). If someone else tried it, look for "case 'article'" in kunena.paser.php (below, here) to configure it for your sites. I haven't tried it with J!1.6, but I made the same edits for both versions.

Feedback is most welcome!

root/component/com_kunena/lib/kunena.parser.php

Also, I hardcoded a 100 word limit on the article text in posts, because the parameter regarding intro vs. fulltext doesn't seem to work.

Here's the other one:

root/plugins/content/kunenadiscuss.php

No configuration necessary on this one.

Interesting code.
The second file (plugin) seems generic and I just have a comment on the fact that the "$multisites_id" is always present and therefore the $siteid will never be empty.

Concerning the first file, I think a generic code could be writtent to replace the hardcoded value.
The DB name and table prefix can be retreived once the site ID is known.