I got mad at them, so I started digging.
I spoke with several people who worked on Buran (at that time, my parents were working at a factory that had designed Buran's instrument panel). Nobody there knew about Drakon. They apparently used a mix of Prolog (its Russian dialect, ПРОЛ), low-level assembly, some Modula-2, Fortran, etc.
It's questionable that the Soviet engieers would have been comfortable working with a graphical language, back when many of them still used punch cards to enter programs.
It could have been used in documentation or in specs, because Drakon indeed is a better graphical notation than the regular flow charts.