Not from the server side, no.
The way a web page submission works:
You submit
Data goes over the wire
Server processes request
Server runs your business rule
Data goes over the wire
Result form renders
There's no way for your server side business rule to "push" data out to the client. All you can do is queue information that will, in turn, be sent out when the form eventually returns (like addInfoMessage()).
Best approach I know of (and it's sub optimal), is to set a hidden field on your form via the business rule, then have an onLoad client script pop a dialog to the end user if it sees data in that field.
