Joanna isn't her understudy so I don't think she would have gone on as Helena. Donna Mig-something (forgive me) has gone on for Patti a few times and I heard was very well received!
Donna is first cover, Joanna is second cover. Joanna would only go on in an event where both Patti and Donna weren't able to perform. Since Patti is almost never out, it's not likely Joanna will ever go on.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body