For those still stuck on 1.8, edge cases left as an exercise to the reader...

hashed_sprintf.rb

require 'test/unit'

class TestHashedSprintf < Test::Unit::TestCase
  def hashed_sprintf(string, hash)
    array = []
    cleaned_string = string.gsub(/%<(\w+)>/) do |match|
      array << hash[$1.to_sym]
      '%'
    end
    sprintf(cleaned_string, *array)
  end

  def test_basics
    assert_equal(sprintf("%3s %2.1d", "foo", 5), hashed_sprintf("%<code>3s %<total>2.1d", :code => "foo", :total => 5))
  end
end