<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <path id="thePath" d="M50 150 L 450 150" />
  <pattern id="pat" patternUnits="userSpaceOnUse" x="0" y="0" width="400" height="400">
    <text x="50" y="100">This text should be ABOVE the other line.</text>
    <text>
      <textPath id="theTP" xlink:href="#thePath">This text should be BELOW the other line.</textPath>
    </text>
  </pattern>
  <rect width="400" height="400" fill="url(#pat)" stroke="black" stroke-width="1"/>
</svg>