xslt nested select from xml node value

I've already reviewed some of the other posts regarding nested selects and don't believe that they address my use case. Essentially I am trying to create a user account in another system through a web service and need to pass a Login ID that originates from a field in my xml which could essentially be almost anything, such as employee id, email address, UUID, etc. The field to use will come from a configuration value that goes into the generation of the xml. I've abbreviated my xml and xslt for simplicity, so please don't suggest that I use a choose or if statement as I need to keep the possible xml fields to choose from wide open.

Sample XML:

<root>
  <General>
    <Name Prefix="MR" First="Mickey" Middle="M" Last="Mouse" Suffix="I" Title="BA" Gender="M" BirthMonth="02" BirthDay="26" BirthYear="1984"/>
    <Email Work="test9999@acme.com" Home="Homeemail@gmail.com"/>
    <EmployeeId>9948228</EmployeeId>
  </General>
  <ConfigProperties>
    <LoginID>root/General/EmployeeId</LoginID>
  </ConfigProperties>
</root>

XSL Sample:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" omit-xml-declaration="no" encoding="utf-8" indent="yes" />
<xsl:template match="/">
  <Response xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <xsl:variable name="xxLI" select="root/ConfigProperties/LoginID" />
    <xsl:attribute name="LoginId"><xsl:value-of select="$xxLI"/></xsl:attribute>
  </Response>
</xsl:template>
</xsl:stylesheet>

Transformed XML:

<Response xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          LoginId="root/General/EmployeeId"/>

What I'm really hoping to get back is something like:

<Response xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          LoginId="9948228"/>

I'm stumped. Any thoughts?

Solution

There is no way to do this in plain XSLT 1, but if your XSLT processor supports the "dynamic" extension (XALAN supports it), you can do this:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:dyn="http://exslt.org/dynamic"
    extension-element-prefixes="dyn">

    <xsl:output method="xml" omit-xml-declaration="no" encoding="utf-8" indent="yes" />
    <xsl:template match="/">
        <Response xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
            <xsl:variable name="xxLI" select="root/ConfigProperties/LoginID" />
            <xsl:attribute name="LoginId"><xsl:value-of select="dyn:evaluate($xxLI)"/></xsl:attribute>
        </Response>
    </xsl:template>
</xsl:stylesheet>

I tested this in Oxygen/XML using XALAN and got this output

<?xml version="1.0" encoding="utf-8"?>
<Response xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" LoginId="9948228"/>