If your rest API directly return the XML and is open for to public access, article already mentions the detail about how to use that:
If the data XML url is not accessible anonymously, you can write your own code, fetch the data in in you jsp or java code and set that in request attribute which would be used for prefill. The above article mentions how to prefill from request attribute and has a sample.
Or is it that you want to prefill part of the form based on some user gesture like lookup address based on zip code?
Thanks for the article. It was helpful.
Would have liked a bit more detail in the example especially around how to invoke the rest API to pre populate the form.