Continuing the discussion from Compose key sequence for a backwards R?:
I didn't find out anything about a compose key sequence, but I finally managed to get a backwards R by keying in the appropriate unicode sequence (which is 042F).
Initially I could not manage to key in unicode sequences, but after much thrashing around I got it to work. One of the problems was that I did not have ibus-daemon on my system. Just in case this might be helpful to others, here, briefly is what I did:
sudo apt-get install ibus
System -> Preferences -> Personal -> Language Support
Keyboard input method system; choose IBus (from IBus, XIM, none).
Now in a terminal window, when I press u I get an underlined "u", as one is supposed to get (which I was not getting before).
Then I can type 042F and bingo! I get Я !!! Success.