Why the need to digitise it though? I was thinking about this earlier and given that it's an analogue signal and you have sync timebases, if you knew the frequency of the sync components (which are part of the spec), then it should be reasonably trivial(!)
For a single scan line, say you have 1024 x 800 LED's the first row would be illuminated in sequence based upon the delta from the sync component.
Then again, I suspect that the POV might not be sufficient for this and that the array would need refreshing during the display of a frame hence the frame store/display would be the more reliable way. Any sync issues with my earlier thoughts would make the timing critical.
Rip apart an LCD projector or TFT and wire that up to LED's
David.