When executing a release:perform, Maven forces hg to perform a clone of the remote repository in the target/checkout directory:
[INFO] EXECUTING: /bin/sh -c cd /home/hudson/Builds/jrawio~TEST/target && hg clone -r 1.5.4 https://@kenai.com/hg/jrawio~src /home/hudson/Builds/jrawio~TEST/target/checkout
Hg repos can be huge (even gigabytes) and thus operation causes a waste of bandwidth (and of time). It seems that the most obvious thing to do is to clone from the local copy in the main directory (e.g. hg clone -r 1.5.4 ..) and if you need to do a laster push you can explicitly specify the URL of the remote repository to the hg push command.