You're quite right, Barbara. That was when "Live from the Met" on PBS was really live, and we didn't know till we tuned in that evening that we weren't going to see Stratas after all.
Migenes wasn't quite jumping in on TV without notice. She had been scheduled for a performance late in the run, and had not only rehearsed, but actually done the previous performance. All the intermission features (good ones) had of course been pretaped and featured Stratas. (Which I suppose is one reason they aren't included as extras on the DVD that was issued a few years back.)
That run was the first time the Met did the complete 3-act Lulu. Berg's widow had suppressed Act III for several decades, and the production had premiered a few years before in the then-inevitable 2-act format, but had been designed so that it could eventually be revived in complete form, whenever that became possible. And so it turned out.