Saxon

For reference, here's how it will work in 9.7:

Collections can now be stable, meaning that multiple calls with the same collection URI are guaranteed to

    return the same results. For conformance with the specifications, this is the default, although it can be

    expensive, because the contents of a collection have to be maintained in memory just in case it is used

    again. Collection stability can be switched off in several ways: the collection URI can include the query

    parameter <code>stable=no</code>; the collection URI resolver can return the property value <code>stable="false"</code>

    in the <code>Properties</code> object supplied to the <code>resolve()</code> method; or the Configuration

    property <code>FeatureKeys.STABLE_COLLECTION_URI</code> can be set to false.</p>

Thanks for the summary; sounds like a sound solution.