Technically, yes, if you have access to the system, but it does mean you have to switch the storage from append only to read write, then pull out the original files, recompute the hashes from the time you did the modifications, and do all the rewriting as you go. For a continuous thing like this your security is only as good as the weakest link, so you could have a problem there, but publishing or sharing the hashes as you go could further protect in this situation. If others have the hashes and then some of them change we know there was a change of data. Add or change something and every hash from then on is different. Seems plausible to make it much better than take my word.