gistfile1.builder

module VarAccessor
  Sass::Script::Functions.send :include, self
  def self.variables
    @variables ||= {}
  end

  def self.set(values = {})
    variables.merge! values
  end

  def variable(value)
    VarAccessor.variables[:"#{value}"]
  end

  def url_for(value)
    "url(#{variable(value)})"
  end
end

VarAccessor.set :background_image => @company.settings.banner.url

gistfile2.sass

#header
  background: url_for('background_image')

I'd add it somewhere in lib and load it in an initializer, for example.

Also, I am not sure this still works with the latest Sass, but it might.