Dear Reader,
I had fixed the issue. Instead of using nullfs to get access to the /usr/home directories, I am using unionfs, which basically does the same for my goals (unless someone corrects me in misunderstanding things) and this does not seem to generate the same issues. Various sites are now running happily behind the WWW Jail. Time to finish my document on how I did setup the entire beast.
Thanks all for listening, helping, and giving tips (Alexander and Miroslav!)
English 
Nederlands
Hi, having the same issue with nullfs, php and wp inside jails. I can see you got a solution, can you reveal some details about it? How you setup, how you mount the unionfs directory structure, since I’m getting into issues with this. Thanks a lot.
I seem to have even more complicated setup, since I’m mounting home from zfs over nfs and then use nullfs to get it into jail as well. Unionfs cannot work on remote mounted filesystems, locally it has no issues. Tricky is, that WP doesn’t like nfs either. What a lame product :/
Hmm Just read this reply. That seems tricky indeed. Did you perhaps contact the mailinglists to ask how such a setup via NFS would possibly work as you wish? I can perhaps poke around a little wrt. nullfs and the wordpress issue but as I do not use NFS in combination with this machine I cannot do a proper job there.
Thanks
Remko