See? Just add a check in the code which implements id() if the
current document (infoset or whatever you call it right now) is
valid. If not, then id() will never work and then you should print
a warning because that is *never* what the user wants (no matter
if he uses XPointer or whatever).

Well, unless every processor implements your suggestions,
use xsl:key and key(), which is independent of the presence
of a DTD and will work just as good.
You might forget about id() after some time... :-)