A reader points me to this link, noting that Waltraud Meier flew in from Germany last night to sing tonight's Isolde. Meanwhile Peter Seiffert is back as Tristan.
I believe Meier has, as the reader pointed out, been avoiding the role here on purpose. But a one-off is surely less risk than a whole run, and she's sung it often with Barenboim before.
This has the makings of an event.