Just to keep those interested in the loop: i got the six displays working by slightly
modifying the timing. Seems like the Matrox default value for the vertical blanking
(1066) was too aggressive for the Acer V223Wbd LCDs and needed to be slightly
increased. In order to compensate that i also slightly increased the pixel clock.
The appropriate modeline looks like this:
ModeLine "5040x1050" 328.80 5040 5104 5168 5376 1050 1053 1057 1073 +HSync +VSync
However, one of the six displays was DOA, so will need to return it ...