The only way I can think of to fix this is to add all the necessary packages as BuildRequires, so that they are available when sphinx tries to import bkr.server code. I have submitted a patch on Gerrit which does this. It builds successfully in dist-5E-eso, eso-rhel-6, and dist-f14 with the extra BuildRequires (and it fixes the sphinx problem).
http://git.fedorahosted.org/git/?p=beaker.git;a=commitdiff;h=660c77974ffb7742150a858c3407dd48176031bb