OK, in -common add "subexercises" to the match on mode="is-xref-target". That
should be all that is needed.
Then I get a good "xref" from an example I added, but I also get from the HTML
conversion (not the LaTeX conversion):
PTX:WARNING: could not translate string with id "subexercises" into language for
code "en-US"
which, of course, can be (tediously) addressed.
BUT - I can't see the place where adding (and enabling) this cross-reference
should be trying to get this string. Which I suspect is some inefficiency (or
worse).
Would you like to go on a wild goose chase? "I give up." ;-)
Rob