I have no problems in LibreOffice, Geany, Terminator… any GTK app to insert Unicode codepoints with the Ctrl-Shift+u unicode + Enter keyboard shortcut. It works well. Here’s an example: (U+1f66)
ὦ
(In Terminator, codepoints with f do not work because of C-S-f keybind, but it is a small price to have that functionality in the terminal.)
But with Sigil, a Qt app, I do not know to make it work.
I created a very simple stupid script to choose an unicode icon. (font awesome list) (It can be used for other fonts too, when create another list of icons of that font)