I don't know how often this actually happens in real life, but I have never seen a woman serve as sandek. In most situations, in normal times, a man will almost always be available, so at frum bris ceremonies, this rule is strictly adhered to. Perhaps it happens in more liberal communities, I do not know.

Corona has done strange things to our Jewish life (and by "strange" I just mean "unusual", not good or bad). We have spoken about zoom minyanim, had porch minyanim, encouraged people to daven at home, have baby namings, bris ceremonies, weddings and bar and bat mitzvahs over zoom, and sadly also funerals and condolence visits. We talked about having guests to the pesach seder via Zoom. And more.

In Russia, CoronaVirus caused a woman to be sandek at a bris.

Rabbi and Rivky Zalkas are the Chabad shluchim in the Russian town of Bryansk. According to Haredim10, Rivky Zalkas tries to call everyone in her contacts list on their birthday and wish them a happy birthday. A couple of months ago she called a young woman who had visited the Bryansk community a year or so prior but had never returned. When she called, the young woman told her that she is pregnant and will soon give birth to a baby boy.

Rivky offered to be available to help in whatever way she might need, especially in assiting to make a bris for the baby. 

This week the young woman called Rivky Zalkas saying she had given birth to the baby and would like to make a bris for him. Her only stipulation was that because of CoronaVirus it must be very small with the only people present being the mohel Rabbi Zalkas and Rivky.  

This past Friday was the bris and because Rabbi Zalkas was the mohel, and the father of the baby is not Jewish, Rivky Zalkas was appointed to be the sandek! 

Would that be called sandekes? sandeka?

The strange things that happen because of CoronaVirus - a Chabad woman acting as sandek!

And, look what one phone call did. It seems likely to me that this young woman was not going to have her baby undergo a bris. Maybe she did not even know about it. Rivky Zalkas calls her up just to say happy birthday and that leads to this woman deciding to circumcise her baby boy!