nosearchable.user.js

// ==UserScript==
// // @name          NoSearchable
// // @namespace     http://d.hatena.ne.jp/javascripter/
// // @include       http*
// // ==/UserScript==

function noSearchable(sentence) {
  var inconspicuous = "./ ,_-~?+";
  return sentence.replace(
    /[一-龠]/g,
    function (m) {
      return m + inconspicuous.charAt(
        Math.floor(inconspicuous.length * Math.random())
      );
    }
  );
}

function insert(node) {
  var p = node.ownerDocument.evaluate(
    "descendant-or-self::text()",
    node,
    null,
    XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,
    null
  ), t, i;
  for (i = 0; i < p.snapshotLength; ++i) {
    t = p.snapshotItem(i);
    t.nodeValue = noSearchable(t.nodeValue);
  }
}

insert(document.body);

document.addEventListener(
  "DOMNodeInserted",
  function (e) {
    insert(e.target);
  },
  false
);