<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" />
<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>
</svg>