Greek letters via compose key

h/t to Peter Williams: https://newton.cx/~peter/2013/04/typing-greek-letters-easily-on-linux/
If you don't have an .XCompose file in your home directory yet, copy the default file (replace locale if necessary, e.g. 'de_DE.UTF-8'):
cp /usr/share/X11/locale/en_US.UTF-8/Compose ~/.XCompose
The file in your home directory should take precedence over the one in /usr/share/… The default XCompose file already has the greek letter mapped using a key (line 5357 in my instance). If you don't have a Greek dead key defined in your key map, you can just switch to a different Compose prefix - e.g. 
<dead_greek> <d> : "δ" U03B4 # GREEK SMALL LETTER DELTA 
to 
<Multi_key> <g> <d> : "δ" U03B4 # GREEK SMALL LETTER DELTA 
which produces δ via <Compose>-g-d. I had to restart my application (e.g. LibreOffice) for the changes to take effect, but needed no logout/restart.
Also, sometimes the damn letters don't show up. It took me a while to realise that this might have something to do with the font in use not having them implemented (*headdesk*)