How to store an XML attribute in a frill register

I am looking at composing using XML as input. We have used classic mode for many years so I am familiar with XPP.
In classic mode I can store text by putting a <px;;#> in the Prestring and <pa> in the Poststring. I have no idea how to do a similar thing in XML mode.
This is a simple example of my XML:

<?xml version="1.0" encoding="utf-8"?>
<library state="PA">
      <title isbn="0-688-15428-X">Gung Ho!</title>
          <corporation>William Morrow and Company, Inc.</corporation>
          <state>New York</state>

What I want to do is to store the “isbn” attribute value in a frill register to be used in a running header.
It must be easy but I just can’t find it in the documentation.
Any and all help will be appreciated.

Barry Frank
Reed Technology
Solaris & XPP 8.4C.1 SP #3