[xsl] using keys on variables

["Graydon graydon@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

In XSLT 2.0 and subsequent, 

key('keyname',object,$variable)

is acceptable but only if the $variable is a document node.

So I have found myself going

<xsl:variable name="temp">
    <xsl:sequence select="$preExisting"/>
</xsl:variable>

So I could use a key on a variable typed as an element.  (In the
specific case, to pull a subset of an already constructed sequence of
middling-complicated mapping elements; A maps to B but there's metadata
about A, B, and the mapping in there, so generating it again isn't
attractive.)

Is there a better way to approach the requirement that the variable be a
document node in order to use the key?

Thanks!
Graydon