I found a half-way fix for this.
I added Options "DPMS" "false" to my xorg.conf Monitor section.
I also changed that line in rc.M to read:
/bin/setterm -blank 0 -powersave off -powerdown 0
This works until I open a window, then the screen blanks out again (even if I subsequently close all windows). This seems to be an issue with the DPMS and not the rc.M line - which did what it is supposed to; it doesn't powerdown the Monitor into a hybernation mode anymore.
So I'm about 1/2 way there, but again I am out of ideas at the moment.