You are after get and set pixels - something a fair few people have been asking for.
As far as I’m aware it isn’t planned for G2 (technical reasons mean it would be prohibitively slow I believe*), but I think there’s various petitions in the new feature area, such as:
http://feedback.coronalabs.com/forums/188732-corona-sdk-feature-requests-feedback/suggestions/3584583-getpixel-putpixel-
But do hunt around for the one with the most votes - several people have all asked for the same thing but with different terminology so there is a bit of fragmentation in there.
*Doesn’t mean I don’t think they should do it, I’d actually like the feature myself!