Many thanks, Hugh and Martin!  This is extremely helpful.

Hugh: Forgive me if I've missed it, but the draft Guidelines don't
seem to indicate where the XPointers can be used.  (Without your
sample code in this email, I wouldn't have known for sure that we
could use them in [log in to unmask])  Is it safe to assume they are kosher
anywhere data.pointer is?

Do you think a concrete example such as this "pg. 8-12" one would be
useful to include in the Guidelines?

My only other comment about the draft is about section 16.7, which
contains slightly different syntax than what's described in the
XPointer section (16.2.4).  See the example at

  <ptr xml:id="rzuiq1" target="./#xpath(//div1[6]/p[1]/q[1])"/>

The "./" in this @target raises the question of "context".  I read "."
as referring to "the current context" (i.e., this ptr's @target), but
I'd be surprised if this was so; it probably means "relative to the
current document root", in which case I'd argue the "./" is
extraneous, and the @target value should begin simply with "#".

Thanks again,