The Up-to-Date Bartender's Guide by Harry Montague (1913, p.41)

1 piecelemon peel — Dropped in.acid
1 jiggerJamaica rum — Substituted for brandy.base
1 teaspoonfulsugar — bar sugarbase
hot water — Substituted for ice.dilution
nutmeg — Grated on top.other
  1. Substitute Jamaica rum for brandy and proceed as for Brandy Sling.
  2. Serve hot with lemon peel and grated nutmeg.

Garnish: Lemon peel and nutmeg

Source is explicitly substitution-based.

See also: Hot Brandy Sling, Hot Rum

Source says to substitute Jamaica rum for brandy and proceed same as for Brandy Sling.