Speaking of toolbar buttons.  Another common practice is to add a toolbar button that will perform some complex operation in JavaScript.  Since you generally don't want the JavaScript itself inside your isv.config.xml file, you need a place to put it where it's easily maintained.  How about in the Form's OnLoad event?
Buttons are added to the [...]

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One of my current projects has requirements to add toolbar buttons to allow the user to create a new, related Entity such as a phone call or appointment with a click of a button.
Buttons are added to the toolbar by modifying the isv.config.xml and inserting code similar to the following:
<Button
Title="Phone Call"
[...]