Thanks, everyone, for the great ideas so far. Keep them coming! I didn't know that such materials existed. To recap suggestions offered via blog or email:

chalkboard adhesive surface
blackboard paint
chalkboard fabric

Then use plastic food wrap to protect any board whose message we want to preserve indefinitely.

Now it's just a matter of selecting a board to use as the "base" that can survive wet conditions for a long time and it lightweight enough to easily carry for a long time.