Hiya,
I think you will need to go about this in a slightly different way. As the person is not yet logged-in there wouldn’t be a way for the system to store the text (without some custom programming or clever work-around) and then allow you to log-in and then re-fill the comments form out for you again.
Your best bet would be this :
1 Show the existing comments.
2 If person is not yet registered or logged-in then show a link that states they need to Register or Log-In before they can comment.
3 User registers or logs-in using links provided and are then returned to the page they were just reading.
4 User can now comment.
That should hopefully get you to what you want. Slightly different but a lot easier than trying to remember the text they have entered somewhere and then re-inserting it to the comments form.
Hope that helps.
Best wishes,
Mark