In C5 it is possible to put in HTML and codes anywhere you want. A XenForo "Box" as an add on would be cool. Then the C5 content can get input from XF ?
I was just looking at C5 yesterday. It looks like a very promising system. I may have to work with it in the future. The only think I don't like is the ACP which I find to be fun to use but lacking in functionality and GUI design. I'm going to keep my eye on it, even build with it a bit with my new VPS server.
Currently however, until C5 improves it's ACP....I think I'll stick to the proven standard that is WP.