Thanks to everybody who spent their time on this issue.
It seems that call
(load-char face char :ft-load-monochrome)
was not sufficient, contrary to my (under)undestanding of FreeType docs. Instead, two calls
(load-char face char :ft-load-default) ... (render-glyph glyph :ft-render-mode-mono)
made what I wanted.
If anyone knows FreeType expert, please ask him about this.
Sincerely yours, Dmitri