There is ticket http://issues.jenkins-ci.org/browse/JENKINS-1871 which has implemented support of setting artifacts permission. But it is implemented as "Feature Controlled with System Properties" and "and subject to change without notice." per http://wiki.jenkins-ci.org/display/JENKINS/Features+controlled+by+system+properties.
So this ticket is to promote this to official feature since that page says "If you find some of those useful, please file a ticket to promote it to the official feature."