Dropping to 1080i made no difference.
The Cat 5's were run and terminated by me...interestingly, across the same pair of Cat 5 cables via a Kramer VGA/Cat5 extender I can get fantastic analogue RGB at 1440 x 900.
The 2nd attempt to an identical Pioneer 50" (otherwise unrelated to the first attempt) over approx 18m distance produced no result at 1080p (although I had the flashing input on screen again) however, reducing the res to 1080i produced a nice pic (albeit at 1080i)
Again cables run and term'd by moi, very confident they're not at fault.
Alas, in both cases, there were 2x full HD displays to drive and I was really counting on the exender to get a 1080 signal to the second display. Even in the case of the sucessful 1080i pic, it was nothing other than exceptionally frustrating because of course it meant feeding the main display a 1080i signal also, unacceptable in my view.
As an aside, I have had sucess with both the 2 in/1 out "auto sensing" switchers I have integrated recently (they were powered).
However a similar product purchased from Jaycar - non auto-sensing was an utter fiasco.
Introducing such add on's to HDMI appears to still be somewhat hit and miss.