I am a very bad man! Matthew Brown's original work for the AHA REF is still languishing on my computer and has not been forwarded for inclusion in the REF information because I have my thumb in my a$$. Fortunately, you can get an indication of Matthew's work from his presentation at the Grand Rapids conference last summer.
As Jeff mentions, the use of a hopback did produce very substantial bittering. However, it needs to be known that Matthew's work included a bittering charge in the hopback during the transfer of the wort from the tun into the kettle. So there was ample opportunity for the alpha acids to isomerize in the kettle and contribute their bittering. So, there were no hops or hop matter in the kettle and there was still substantial bittering.
Yes, Matthew also did a split of the batch and used the hopback again to post-boil hop half the batch. Those at the conference presentation tasted both beers and they were good.
A very interesting use of a hopback.
Yes, I'll un-thumb myself and get that research article on its way to AHA.