I think the idea of running as a user other than root is good, but
  having venus run as 'nobody' would make the venus file cache visible
  to any other process running as 'nobody'.  I don't think that would
  be very nice!

true - I was being sloppy, and really should have talked about a
uid/gid just for venus.  But that seems far away, and starting to
remove the reasons why it can't be done one by one needs doing first.