hi..this was my requirement also..actually this kind of message we need for shoing confirmation and , depending upon the value action , code will be excuted from next line...as like VB/COBOL/Delphi etc...
But i don't think so it is possible, so in action oncomplete we have used. and from where we want to sshow the message in code , creating a flag. No on action complete checking that flag , if its true showing the message modal panel. there in button action binding executing the code to be called next.

you could manage it with showWhenRendered - if this attribute is true - modal will be shown after page rendered or component reRendered