In my tweaks & tests today I noticed that even if I disconnect ROG & CLK lines from the arduino I still get the same (first pixel) waveform as with them connected so they seem to have no effect.
Maybe CLK is not running quick enough though I cannot go much faster as it takes 24uS to analogue sample and store a pixel and with CLK running at 64uS PP I only have 8uS overhead when sampling.