Martin, I would suggest a slight change to your code:

B <xsl:next-match/>

... rather than:

B <xsl:apply-templates/>

... and when the heavy-lifting code is not being imported, then put a
priority="1000" on the suggested match.

That will allow Wolfhart's stylesheet also to support a template rule
for the root node.

Agreed, that is a better way, should have thought about it probably as
it is an easy way in XSLT 2 and 3.