Historians now know what happened to heroine Betsey Doyle after the War of 1812. According to the Buffalo News article [Update December 20, 2012: bad link - try this one*]:
A nearly 200-year-old missing-person case has been solved by the Niagara County historian’s office. Doyle was lionized for hauling red-hot cannonballs to gunners at Fort Niagara during a November 1812 cannon duel with the British forces in Fort George on the other side of the Niagara River.
*Emerson...recently found documents...that proved Doyle escaped the fall of Fort Niagara in December 1813 and made a winter trek of about 400 kilometres — with her children in tow — to a U.S. military camp near Albany, where she worked as an army nurse until her death in 1819.