Hello Bearsomg,
I have been trying the exact same mod as yours (to include a mini ITX-PC board in an iMac G3 slot-loading while keeping its PSU and CRT !) and I am now stuck exactly where you seemed to be stuck on Sept.2012 !
I have made a VGA cable to connect the HE20 connector to the VGA output of the PC board but it looks like I am still missing some pins.

Even when shorting +5V-trickle (T5V) to PFW, I cannot hear the "PONK" sound of the CRT charging up ....
Have you made yours working right finally ?
Could you finally find if pins 4, 5, 7, 8, 12, 19 and 20 have to be connected and where to ?
Thanks for any help you can share here or by PM.
ym58