Is scanning an handwritten peer-review suitable for editor/authors?
I do not think that there is anything wrong with a scanned handwritten report. As for anonymity: you may even sign your referee report as it is your decision to stay anonymous or not. As for usefulness: I once received an annotated scanned manuscript as a referee report at is was tremendously helpful (as there were several suggestions for formulations which greatly increased the readability). A computerized summary would also be helpful for the editor to form the decision and also for the authors to find out what you main points are (so that they can distinguish between just typo corrections and serious remarks).