At least in 5.5 it's still not available. You have to call in to support and they can do it through root access but they make it difficult to get accomplished. The file to be modified is /etc/network/interfaces . I managed to get a screen capture when they did mine, but I sanitized the ip addresses for your viewing pleasure. When completed it should look something like this.
# /etc/network/interfaces auto generated by LEM firstboot
# Generated at Fri Feb 1 11:48:03 2013
auto lo eth0
iface lo inet loopback
iface eth0 inet static
up route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.1.253
Not currently, but we've had a feature request for this. You might check the Log & Event Feature Requests forum to see if there's an idea there already you could upvote, or create one so that we can track Thwack votes.