The ebuild from jail puts jail.conf in /etc/jail.conf and mkjailenv looks for it on /usr/etc/jail.conf and fails. A simple link solves the problem.
the better answer is to fix/patch the source cod since /usr/etc violates FHS
Totally agree, perhaps mailing the author pointing this out would also be a good ideia.
Our ebuild doctors up a bunch of files, so complaining to the author is probably not something we need to do. In the mean time, I released -r1 which fixes this path problem. :)