The easiest way I can think of would be to rewrite the core in qa-page-ask.php:
qa_redirect(qa_q_request($questionid, $intitle)); // our work is done here
and add a $_POST parameter to the redirect:
qa_redirect(qa_q_request($questionid, $intitle), array('notice'=>'test');
Then have your layer check for the request:
and find some way to output the text (check the badge plugin for a SO type of notification).
Otherwise, I assume 1.5 will have some way to interrupt the redirect (maybe you can add this to the list of pluggable functions...)