A few years ago I thought long and hard, tried quite a few different things, and ended up settling on that sequence. And with this in my ~/.XCompose:

  <Multi_key> <h> <r> : "—⁂—"

… I can get it by typing `Compose h r`. (“hr” as in <hr>, HTML’s Horizontal Rule.)

At one point since then I came across another person on HN that had independently settled on the same three characters (em dash, asterism, em dash).

In my personal dialect of the lightweight markup language I’ve been making, —⁂— is even the syntax for a horizontal rule. :-)