I have re-started the design from scratch, because I could not get DDR memory to work reliably (DDR can't cope with changing frequencies and is very sensitive to noise - datastrobe PLLs and 2.5V technology).
I've gone back to SDR memory, but increased the bus width to get to the same data rate (32 instead of 16 bits). Memory chip is a x32 type, saving lots of space on the board (you may know that the old version of Indivision AGA had two memory chips, but this one will only have one). Memory performance is still high enough to saturate the 165MPixel/s maximum pixelclock of the DVI encoder: Total memory performance easily exceeds 200MPixel/s (old Indivision AGA: about 105MPixel/s).
The monitor connection is made with a 2x10-pin (1.27mm by 2.54mm) header using 0.635mm twisted-pair ribbon cable. Still haven't decided if it'll be a custom cable or a small board with header - depends on availability of parts when mass-production is due.