These steps do not reproduce the problem.  I built the Nevow egg using
"easy_install Twisted Nevow".  The subdirectory I used was 't' rather than
'FOO', but I don't think that's relevant.  For that matter, I don't see how the
given steps could possibly produce the error described, unless zope.interface
was doing a require('Nevow'), and importing twisted caused that to happen.