I use an alternate form that doesn't require alignment to an arbitrary column and works with tab indentation without needing to mix:

  thing = {
  [→][→]some_key_1: some_var_1,
  [→][→]some_key_2: some_var_2,
  [→][→]some_key_3: some_var_3,
  [→]}

Several benefits: easier to insert lines at the head or tail without making the SCM diff ugly, also avoids merge conflicts in those cases, and I use indentation of two levels to separate it from code.

Why suffer a problem when it's easily avoided? :)