denvar, envars in your data store.

denvar.rb

module Denvar
  def self.lookup key
    @lookup.call key
  end

  def self.activate! &block
    @lookup = block

    class << ENV
      alias :orig_lookup :[]

      def [] key
        orig_lookup(key) || Denvar.lookup(key)
      end
    end
  end
end

usage.rb

store = {"BUNNY" => "HOP"}

Denvar.activate! do |key|
  store[key]
end