AltGr-SPACE (for _) doesn't trigger melisma in lyrics
At least 2 keyboards. the french BÉPO ones, code UNDERSCORE by AltGr-SPACE, see https://bepo.fr/wiki/Accueil
However, this typing in lyrics mode reacts as a simple SPACE (at least in my 16.04 Ubuntu...perhaps a Ubuntu specific problem, see https://bugs.launchpad.net/do/+bug/379575.)
I workaround it by switching AltGr-SPACE and AltGr [alpha key for 8] in the configuration file of the keyboard.