Example of how to interpose yourself in a message stream.

ipc/ipc_channel_proxy.cc

Index: ipc/ipc_channel_proxy.cc
===================================================================
--- ipc/ipc_channel_proxy.cc	(revision 83739)
+++ ipc/ipc_channel_proxy.cc	(working copy)
@@ -281,7 +281,8 @@
                            Channel::Mode mode,
                            Channel::Listener* listener,
                            MessageLoop* ipc_thread)
-    : context_(new Context(listener, ipc_thread)) {
+    : context_(new Context(listener, ipc_thread)),
+      outgoing_message_filter_(NULL) {
   Init(channel_handle, mode, ipc_thread, true);
 }
 
@@ -290,7 +291,8 @@
                            MessageLoop* ipc_thread,
                            Context* context,
                            bool create_pipe_now)
-    : context_(context) {
+    : context_(context),
+      outgoing_message_filter_(NULL) {
   Init(channel_handle, mode, ipc_thread, create_pipe_now);
 }
 
@@ -340,6 +342,9 @@
 }
 
 bool ChannelProxy::Send(Message* message) {
+  if (outgoing_message_filter())
+    message = outgoing_message_filter()->Rewrite(message);
+
 #ifdef IPC_MESSAGE_LOG_ENABLED
   Logging::GetInstance()->OnSendMessage(message, context_->channel_id());
 #endif