Hmmm. I'm still thinking that RGB/Normal pin does something. I'm assuming you connected it each ground to see what happens.
I'm not sure if you need to use that blanking return input or not. I'm thinking you do.
The H-sync and V-sync return might also need to be grounded.
It could still be a number of things. Try also grounding the composite video grounds, or the audio select pin. Sometimes monitors can be funny things and expect certain pins to have a signal or be grounded. I've seen tv's and vcr's that wouldn't accept a video signal unless you plugged an audio cable into the audio ports, even if there wasn't a signal coming through them.
Also, you might have to ground some of these pins back to the XBR's ground instead of the VGA ground.
And finally, and don't try this yet until you run it by the guys I'm about to mention, but you might be have to connect that +5v pin to the RGB/Normal pin. Like I said, don't do this someone who knows about it tells you.
There are two other highly knowledgeable (much more so than me) members here who can maybe help you. Mon Mothma and Sailor Sat. If you sent them a polite message I'm sure they'd give it a try.