Yes, this is manageable.

1. Add a field  named "Restricted", type boolean, and disable "visible", so that column won't appear in a default grid. Enable required and set the default value to false.
2. Generate a XSL-Stylesheet.
3. Edit the generated XSL: look for
   
     <xsl:template match="udt:Data" mode="list">
        <tr class="Normal">
            ....
           </tr>
      </xsl:template>
   
   and change it to
   
     <xsl:template match="udt:Data" mode="list">
       <xsl:if test="udt:Restricted_UDT_Original = 'false' or
                             contains(/udt:Context/udt:UserRoles, ''|Subscriber|">
           <tr class="Normal">
            ....
           </tr>
        </xsl:if>
     </xsl:template>

Sorry, Sebastian is wrong.

udt:Context is part of UDT since UDT 3.3., (Documentation P.46, chapter "Advanced XSLT)

Have inserted the suggested code using a simple table layout and am getting an "XSL Tranformation failed" error.  The template i have made has 3 columns - Name, Description, and for test purposed, the restricted column.  The template I have constructed is:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:udt="DotNetNuke/UserDefinedTable">
  <xsl:output method="xml" indent="yes" omit-xml-declaration="yes" />
  <!--
  This prefix is used to generate module specific query strings
  Each querystring or form value that starts with udt_{ModuleId}_param
  will be added as parameter starting with param
  -->
  <xsl:variable name="prefix_param">udt_<xsl:value-of select="//udt:Context/udt:ModuleId" />_param</xsl:variable>

  <xsl:template match="udt:Data" mode="list">
    <xsl:if test="udt:Restricted_UDT_Original='false' or contains(/udt:Context/udt:UserRoles,'|Subscriber|'">
      <tr class="normal">
        <td>
          <xsl:call-template name="EditLink" />
        </td>
        <td>
          <xsl:value-of select="udt:Name" disable-output-escaping="yes" />
        </td>
        <td>
          <xsl:value-of select="udt:Description" disable-output-escaping="yes" />
        </td>
        <td>
          <xsl:value-of select="udt:Restricted" disable-output-escaping="yes" />
        </td>
      </tr>
    </xsl:if>
  </xsl:template>

  <xsl:template match="/udt:UserDefinedTable">
    <xsl:variable name="currentData" select="udt:Data" />
    <table>
      <xsl:apply-templates select="$currentData" mode="list">
      </xsl:apply-templates>
    </table>
  </xsl:template>

  <xsl:template name="EditLink">
    <xsl:if test="udt:EditLink">
      <a href="{udt:EditLink}">
        <img border="0" alt="edit" src="{//udt:Context/udt:ApplicationPath}/images/edit.gif" />
      </a>
    </xsl:if>
  </xsl:template>
</xsl:stylesheet>

What have I done wrong?

Thanks.