Hi Paul,
The People Picker is a special control but you should still be able to set a rule on it and trigger some actions based on input. Of course, the user filling out the form will have to tab out of the field to have the rule fire. If that is unacceptable, why not just use your own fields to populate users from Active Directory? We've got lots of blogs on how to do that using our Web Service. Probably a way to hook into People Picker too, but I don't know offhand. Worth looking at the object model in Visual Studio though. Maybe there are some methods and properties on the People Picker OnAfter change method. ...
Sorry couldn't be more enlightening,