Kevin,
You have access to the entire XFAModel when customising a Guide in Flex and access the data using the same SOM expression you use in a LiveCycle PDF.
For example, if you LiveCycle form / data structure was like this:
Blah
You can access this by the following (Xfa.instance is a static property - i.e. you don't have to instantiate the object):
--------------------------------
var currentXfa:Xfa = Xfa.instance;
var myFieldObject = currentXfa.resolveNode("rootForm.myFirstPage.myField");
if (myFieldObject.rawValue == "") {
myFieldObject.rawValue = "aNewValue";
}
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In answer to the second part of the question, anything you update in the XFA model in Flex will update the underlying PDF form / data too and vice versa (a script executed in the PDF will update the XFA model in Flex and these changes will be reflected in the Flex app fields). This is achieved by the "Adobe LiveCycle Forms Bridge" (you can see the source if you drag and drop it onto the form from the Custom library palette).
Hope this helps,
John.