> How can I change these values programatically? The only example I've
> ever seen of manage_changeProperties in within a method of the
> propertysheet as such:
>
> propertysheets.myPropertySheet.manage_changeProperties(REQUEST)

You can try this from an external method:

myObject.propertysheets.myPropertySheet.manage_changeProperties(self.REQUEST,myProperty=myValue)

From a python script you just have to change self for context