The chrome.screenlockPrivate API allows select apps to control the ChromeOS ScreenLocker.

chrome/common/extensions/docs/examples/api/screenlock/weblocker/webunlock.py

+#!/usr/bin/env python
+import SimpleHTTPServer
+import SocketServer
+import socket
+import urlparse
+
+VALUE = 'HELLO'
+FORM = '''
+<form method=post action="http://scrap.mtv.corp.google.com:54329/">
+<input type=text name=v id=v value="%s">
+<input type=submit>
+</form>
+<script>document.getElementById('v').focus();</script>
+'''
+ALLOW_ORIGIN =  'chrome-extension://begdogjjbmiahnbahpidlnbgpbjbpajl'
+HOSTPORT = (socket.gethostname(), 54329)
+
+class Handler(SimpleHTTPServer.SimpleHTTPRequestHandler):
+  def do_GET(self):
+    global VALUE
+    self.send_response(200)
+    response = FORM % VALUE
+    mime = 'text/html'
+    if self.path == '/value':
+      response = VALUE
+      mime = 'text/plain'
+      self.send_header('Access-Control-Allow-Origin', ALLOW_ORIGIN)
+      self.send_header('Access-Control-Allow-Methods', 'GET')
+    self.send_header('Content-Length', str(len(response)))
+    self.send_header('Content-Type', mime)
+    self.end_headers()
+    self.wfile.write(response)
+
+  def do_POST(self):
+    global VALUE
+    q = urlparse.parse_qs(urlparse.urlparse(self.path).query)
+    q.update(urlparse.parse_qs(self.rfile.read(int(
+      self.headers['Content-Length']))))
+    VALUE = q.get('v', [VALUE])[0]
+    self.send_response(302)
+    self.send_header('Location', 'http://%s:%d/' % HOSTPORT)
+    self.end_headers()
+
+if __name__ == '__main__':
+  SocketServer.TCPServer(HOSTPORT, Handler).serve_forever()