I'm interested in this too!
So far, I'm mostly getting to know how things are and what to do.
Will post an update if I can make any progress (I'm even still running stock recovery/kernel/system though).
EDIT 1: could get on this a bit. As far as I could see from the *latest* oficial dualboot.sh the idea is to install CWM recovery for the device (I think 18.104.22.168 should be OK for our I9300) and then install SuperSU and the Ubuntu Installer.
Still don't know how the recovery partition and /data/ubuntu get populated though.
EDIT 2: looking at https://wiki.ubuntu.com/Touch/DualBootInstallation
confirmed above statement and also got to know the Ubuntu Installer app is the "actual" installation responsible.
1) What about it under Android<4.4? Is this a requirement for the Ubuntu Intaller APK only or something else imposes this?
2) How is it to get the Ubuntu's recovery.img and /data/system.img images for i9300? Any of you guys could share them for me to try a manual installation?
EDIT 3: Regarding second question above I think I should flash from CWM or TWRP astronfestmon's file:
and then kinda "trusty-preinstalled-phablet-armhf.zip" image that I'm not able to find. Should I use this one instead: