OK, so I will reply to my own post, a concerted amount of luck and resourcefulness on FamilySearch and I have found her ... and she REALLY did not want to be found:
After Thomas T left for Australia sometime after 1902, Eliza (Bennett) Seller eventually ended up marrying Alexander Williamson Armstrong in Jhb in 1914.
On her marriage certificate she lists herself as Eliza Bleach (mother's first husband's surname, died before she married Eliza's father, Jonathan Bennett) Burns, with an affidavit stating that she was a widow.
She seems to have stuck with the fiction that her maiden name was Bleach until her death in 1923, where she is listed as Eliza Bleach Armstrong.
Eliza Bennett and Thomas T Seller were quite a couple of interesting, resourceful people, I think they will each get a page of their own on my family history website, now if I could only find a photo of Eliza!