I'm currently trying an install with a tweaked version of the installer (Sorry paul, been hanging out to play with this since the devs at work pointed me to the demo video ;o) );
Initially I tried using the packages from already d/led ISO option but it was still trying to install from linuxmce.com.
So I pulled the installer apart, initially I tried dumping the iso into /var/linuxmce where it expects to find it after download but it doesn't check if the file exists prior to trying to pull it down so that didn't work.
Second aproach was to tweak the wget-wrapper.sh script to pull the iso from a host on my local network and use that rather than the one from linuxmce.com, so far so good but I haven't got to the actual "install" phase yet (39% of the way pulling the ISO via HTTP).
If you'd like to try yourself when the installer is running it creates /tmp/selfextract.XXXXXX (where XXXXXX is a random string), grab a copy of that while the installer is running and you can play with the backend scripts (just make sure you launch mce-installer from the directory with your modified scripts in it.)
EDIT: Install has still failed, I've still yet to read the install log but I noticed it was still trying to pull the Package list from linuxmce.com, there were also an assortment of database errors, generally relating to pluto_main not exisiting, last log entry is "Package pluto-orbiter failed"
EDIT2: Upon further analysis it appears that the Core install was successful (although I had chosen a Hybrid install). Despite the errors in the log all (from what I can tell) the databases were created properly, the web interface mostly works (at least it seems to, there are a few pages with PHP errors though.), if the new installer hasn't been released tomorrow I'll have a look to see if I can bludgeon it into behaving itself...