Hi David!

You are much better at handling keys than I am. But if I skip the key
bit, and just put an XPath on the select= of the <xsl:value-of> child
of <pointer> in the hm28x template, I think the following XPath does
what you want.

   preceding::pb[parent::*[local-name(.) = $my_hm_local-name]][1]/@folio

(Obviously I set $my_hm_local-name() to local-name(.) earlier in the
template :-)

And it runs pretty quickly using Saxon-HE 9.2.1.1J:
 real   0m2.996s
 user   0m4.634s
 sys    0m0.256s

I hope this helps ...