My original suggestion for comment editing, way back when on LJ, included keeping the text of the original comment on the database. I designed the spec to get it through serious objections from some on the then LJ staff who thought comment editing in general was too great an invitation for drama, hence I put huge numbers of safeguards in that I didn't necessarily want but that would ameliorate the objections.

Despite all this, it took ages to push it through, and as far as I know the assumed drama that would be endemic never happened.

Howeever, keeping copies of the original text was, apparently, not technically possible without a major redesign of the database structure itself, so that aspect of the safeguard was dropped.

Which is, I think, partially a shame, as of all the bits I said would be ueful, that was one I actually liked.

Essentially, your idea isn't possible.