To Ben:
The cache-append idea was my first thought, but I couldn't think of a

way to pull together how to have each request be unique. It's possible
using node-set(), though:
...
...
    <xsl:choose>
      <xsl:when test="function-available('exsl:node-set')">
        <xsl:value-of select="document(concat('/cgi-bin/rand.pl?',

'&x=', generate-id(exsl:node-set($seed))))" />
      </xsl:when>
      <xsl:when test="function-available('msxsl:node-set')">
        <xsl:value-of select="document(concat('/cgi-bin/rand.pl?',

'&x=', generate-id(msxsl:node-set($seed))))" />
      </xsl:when>
...

I tried your solution and after replacing '&x=' with '+' it directly
worked on my local web server!
(I did not know that generate-id works in a browser)

But as already posted it did not work after installed on my real webserver
because that does not have enough power to handle the six document opens.

Btw, I really like the solution of David Carlisle to provide exslt:node-set
for IE browsers (and not having to deal with exslt/msxml):
http://dpcarlisle.blogspot.com/2007/05/exslt-node-set-function.html