EDIT :
I want to use the default processing message feature provided by datatables but looks like it works only with server side processing. I am not using
So I firebugged the example and there is a hidden div on the page that contains the "Processing..." text and the way I imagine display of it is controlled is right before the ajax for the server processing is sent, it shows it and when it's completed, it hides it. I would check to see if you have a default hidden div on your table when enabling bProcessing and control its visibility when the conditions for it appearing are met.
The id of the hidden div should be the ID.OF.YOUR.TABLE_processing
So now I'd create handlers for the different conditions that you need to account for much like the examples seen here:
http://datatables.net/release-datatables/examples/advanced_init/events_live.html