Thanks - I have gone ahead and initiated the chargeback with AMEX.

FWIW, we've only had issues with 2 smaller hotels/chains on this trip we planned. I have about $1600 in hotel stays between the two of them - the second one is through Expedia who hasn't yet extended their flexibility beyond April 30. All of our activity vendors, museums, etc (who presumably are in the same boat economically) have refunded us without us even asking because they're ethical, and presumably know it's not right to hold onto money for a service they can't provide while closed. Good on them.

Like I said before, this experience has soured my views on smaller properties. There's a snowball's chance in hell I will book a nonrefundable rate in the future with any of them, even though this is a fairly exceptional event. As I'm planning our future trips, I've been selecting the major chains whenever possible. You live and learn.