Looks to me like you’ve already implemented it ;).
So are there now three things in this thread - all of which I would love to see -
- Narrative report layout editor
- Freezing report snapshots
- Saving ad-hoc reports (this would be my personal top priority out of the 3).
IMHO each would be useful on its own and more powerful together. Freezing sounds hardest to build, though it can be worked around by exporting to PDF. But where it would become super powerful would be if you could turn the frozen snapshot into a baseline for later comparison. So that’s feature idea #4 :).