Each glyph of an sf::Text is really a quad, not an actual character geometry. This means that we can only specify the texcoords of the corners, which we already use to map the actual character in the font texture.
In conclusion, that effect is impossible with the current API, and without a major break in the way that sf::Text and sf::Font work..
Maybe you could achieve it with a pixel shader testing the text geometry by its color and assigning a texture coordinate?