I finally found a reference that names who the person on the back of the new $50 note is supposed to be... Nehanda Charwe Nyakasikana, aka Mbuya Nehandra
A female hero of the “Chimurenga,” or what is now called the “First Chimurenga,” which was the war of the native Africans to fight the British South Africa Company in an attempt to stop the colonialization of Rhodesia / Zimbabwe. She was a spirit medium and spirit leader of the Shona. She was captured and executed in 1898. It is believed she was born around 1840 and she would have been 45-50 at the time of her death. In the lead up to and after independence (the war for independence became the “2nd Chimurenga“) she was honored by Zimbabweans with statues, street names, naming hospitals, songs, novels and poems.
I'm wondering what they based the image on. I can't imagine there are many surviving photos or portraits of someone who was born, lived, and died in pre-colonial Southern Africa, but I could be wrong about that. By 1890 photography was getting more accessible.
Wikipedia just shows the one image: It looks like there are a LOT of statues of her in Zimbabwe based off this one image but it does look like there are others.