Thanks for the supportive responses to my question about the toddler
resting his top teeth on mom's breast while nursing.  A couple of people
asked me to post the responses I got, so here's a summary:

1) by far the most common suggestion was to have the little one tilt his
head back while nursing (i.e. be positioned slightly lower than the
nipple so that some of the pressure from his top teeth would be
relieved).  The mom reports that this has been helpful to some degree.

2) holding the breast tissue immediately above/in front of his top teeth
down, so the pressure from mom's finger relieves the pressure from the
teeth

3) constantly evaluate/reevaluate the toddler's position: tummy to
tummy, open wide, well-supported, not in acrobatic positions, etc.

4) ruling out thrush, especially since the latch changed when the child
was sick.  There were no antibiotics involved here (in fact, he's never
had them); mom and child did have thrush soon after he was born; it took
a long time to resolve but there haven't been any signs since.
Certainly mom is more prone during pregnancy, but that seems to be the
only predisposing factor (neither eat much sugar, generally good diets,
no antibiotics, etc).

We've talked a lot about the pregnancy-related issues that must be part
of this situation, and I really appreciate everyone's feedback.  All of
these posts were sent to the list, so I won't repeat them here.  The mom
understands at this point that resolving the teethmark issue (if
possible) probably won't take care of all her discomfort, but she seems
OK with this.

I've found this discussion on pregnancy and nursing, as well as the one
on weaning, very interesting.

Thanks for all the support and suggestions -

Morgan Kennedy Henderson
LLLL, Independent Childbirth Educator
Wellesley, MA