Hi,
I assume you are using Personas 3.0 SP2 and you are correct in that you can assign scripts to standard buttons, even toolbar buttons.
One thing to bear in mind if you assign an onClick script to a standard button is that you can suppress the standard action behind the button by returning a boolean "true" from your script. If you do not return a boolean true the standard action is executed after the script execution. Now, if your script navigates to a different screen and stays there then you must return true to suppress the standard action because the standard action will fail (and most likely crash the client) due to the fact the button tied to the standard action to be executed isn't on the screen anymore.
Check out my KB article on this: http://wiki.scn.sap.com/wiki/display/Img/Available+Events+in+Scripting+-+SP02
Best Regards,
Clemens