I have added some enhancements to the XSLT to include page property, query string or cookie data along with the email.. If anyone interested see below

1. Add a new Doc Type "Pliable Hidden" under "Pliable Fields" and add the following properties
   i. Name: Label, Alias: label, Type: Textstring
   ii. Name: Field Name, Alias: fieldName, Type: Textstring
   Allow this Doc Type under Pliable Form
   
   
2. Open PliableForm.xslt and add the following code under renderForm template just inside the first <xsl:choose>

<xsl:when test="name() = 'PliableHidden'">
             <xsl:variable name="propertyName" select="./fieldName" />
                 <input type="hidden" id="{$prefix}_{@id}" name="{$name}">            
                    <xsl:attribute name="value">
                     <xsl:choose>
                         <xsl:when test="starts-with($propertyName, '#')"><!-- Query string --><xsl:value-of select="umbraco.library:Request(substring($propertyName, 2))"/></xsl:when>
                         <xsl:when test="starts-with($propertyName, '%')"><!-- Cookie --><xsl:value-of select="umbraco.library:RequestCookies(substring($propertyName, 2))"/></xsl:when>
                            <xsl:when test="starts-with($propertyName, '@')"><!-- umbraco property --><xsl:value-of select="$currentPage/@*[name()=substring($propertyName, 2)]"/></xsl:when>
                          <xsl:otherwise><!-- page field --><xsl:value-of select="$currentPage/*[name()=$propertyName]"/></xsl:otherwise>
                        </xsl:choose> 
                  </xsl:attribute></input>
            </xsl:when>

Now you can add a Pliable Hidden under a Pliable Form in the content section and use 

- Use document type property name alias as it is. 
- Use @ for built in Umbraco fields.  (eg:- @nodeName )
- Use # for query string.  ( eg:- #id )
- Use % for cookie value. (eg:- %language )