Delegate a key to XCompose, a.k.a. the Multi_key. I have these lines in my ~/.Xmodmap:

keycode 108 = Multi_key clear mod1 add mod1 = Alt_L

Optionally you can have a look and modify the mappings in NP/Unicode.hs new symbols or change shortcuts.

Then, generate the .XCompose file with genXCompose.sh. This script automatically links the generated .XCompose file to your $HOME.

Moreover if you want it to be run automatically you could have a look at 'mr' http://joeyh.name/code/mr/ and setup a fixups hook:

[.XCompose.d] checkout = git clone git@github.com:np/xcompose.git .XCompose.d fixups = ./genXCompose.sh