I'm hazy as to the current status of the saxon:next-in-chain attribute.
Back in 2006, someone noted that it was rejecting all URIs [http://permalink.gmane.org/gmane.text.xml.saxon.help/5961].
I notice that this extension is no longer listed on the Saxon Extension documentation pages, but the processor recognizes the attribute, but it still appears to always throw an error.
Description: saxon:next-in-chain value cannot be specified dynamically
Start location: 20:0
I received this error with the following call:
<xsl:result-document href="MasterTree.xml" saxon:next-in-chain="MasterTreeBuilder.xsl">
"A false conclusion, once arrived at and widely accepted is not dislodged easily, and the less it is understood, the more tenaciously it is held." - Cantor's Law of Preservation of Ignorance.