Example of how to interpose yourself in a message stream.

ipc/ipc_sync_channel.cc

Index: ipc/ipc_sync_channel.cc
===================================================================
--- ipc/ipc_sync_channel.cc	(revision 83227)
+++ ipc/ipc_sync_channel.cc	(working copy)
@@ -378,7 +378,8 @@
           channel_handle, mode, ipc_message_loop,
           new SyncContext(listener, ipc_message_loop, shutdown_event),
           create_pipe_now),
-      sync_messages_with_no_timeout_allowed_(true) {
+      sync_messages_with_no_timeout_allowed_(true),
+      outgoing_message_filter_(NULL) {
   // Ideally we only want to watch this object when running a nested message
   // loop.  However, we don't know when it exits if there's another nested
   // message loop running under it or not, so we wouldn't know whether to
@@ -400,6 +401,9 @@
 }
 
 bool SyncChannel::SendWithTimeout(Message* message, int timeout_ms) {
+  if (outgoing_message_filter_)
+    message = outgoing_message_filter_->Rewrite(message);
+
   if (!message->is_sync()) {
     ChannelProxy::Send(message);
     return true;