In emacs, c-x 8 RET prompts you for unicode character names (or hex) so for rare use you can just spell it out. There's also C-x 8 _ m for em dash and C-x 8 _ n for en dash. (Hit c-x 8 c-h to get a full list of those bindings, like any normal secondary map - they're about as idiosyncratic as the XCompose bindings, but you might find some of them "stick" in your head better (I personally like "C-x 8 1 / 2" better than "Compose 1 2" even if it's a lot more typing...)