I found this on netzwolf.info:
You can enter arbitrary Unicode characters (more precisely code points) in X11 on Linux, if you know their Hex-Code:
- Press Shift-Ctrl (keep them pressed)
- Press also the letter u
- Release the Ctrl-Key
- Release the u-key
- Keep the Shift-key pressed
- Enter the Hex-Code of the Code point with the number of hex digits needed
- Release the shift key
Let’s try with the Cyrillic Alphabet, more precisely with its Unicode Block:
- Ѐ U+0400
- Ё U+0401
- Ђ U+0402
- Љ U+0409
- А U+0410
- Б U+0411
- В U+0412
- Г U+0413
- Д U+0414
Not very fast, but it works quite well for a few characters. Just open the table in one window and use this key sequence in another one.
For frequently used characters it is a good idea to remap the keyboard with xmodmap.
