Where would I start for a module which moves items by tag?

On the server side it's pretty easy to move the instances. The code would look something like this (off the top of my head):

// Walk the album
foreach ($album->children() as $child) {
  // Walk the item's tags
  foreach (tag::item_tags($child) as $tag) {
    if ($tag->name == "THE TAG I CARE ABOUT") {
      // Move the item
      $child->parent_id = $new_parent_album_id;
      $child->save();
  }
}

For a simple UI to get started, you'd need an album picker of some kind.. can't think if we have one of those offhand, but you could bastardize one of the album tree modules to get that. And you'd need a tag autocomplete box, which you can crib from the tag sidebar block. Combine those into a dialog and put it in the context menu for your current album and you're good to go.

This is an N^2 walk in PHP so it's slow, but for small data sets it'll be plenty fast enough. For a more efficient solution you'd want to do a single query that joins the item and the items tags tables to find just the items you care about.
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