I found two tricks around it:
1. Agree to the unspecified seat, go to checkout, and then click on "change my seat". That should bring you to the full seat map
2. In the url, replace "best-available" with "seat-selector". That will take you straight to the full seat map. For instance, the url for the May 29 Bayadere should look like this: https://www.metopera.org/seat-selector?perf=15043#/
This seems to work consistently on my tablet but not on my phone.