<input value="<script src="{$themePath}/scripts/link-parsed-variable.js" type="text/javascript"><xsl:text> </xsl:text></script>" name="uP_fname" type="hidden" />

I want to put a javascript document.write within the <input value=""...

Can't you do something like this:
<input name="myinput" value="" .../>
<script>
  myinput.value = "myvalue";
</script>

Cheers,
Geert