Ok I think ive got this sussed now, looking at the scan doubler on aminet that you mentioned i think I know what it does, it doubles the sync yes but it times the writing of the colour data to the new sync pulse - thats where the timing comes from - now I understand what you are saying.
Looking at this circuit also - if you removed the output stages and converted the output to a digital signal using a 3 channel ADC you could then store the data to RAM on change of the timing pulse resulting in a frame (line) being in the devices ram.
I wonder how well flicker fixing would go if you read line 1 then line 2 while writing line 1 and so on, this would result in only a small a ram being required. duel ported ram could be used so that it could be written to on odd cycles and read from on even ones.
Just a thought