content_for in sinatra : http://proccli.com/railslike-contentfor-sinatra

layout.erb

<div id='sidebar'>
  <%= section(:sidebar) || partial(:default_sidebar) %>
</div>

sinatra_content_for.rb

module Sinatra::Helpers
  def section(key, *args, &block)
    @sections ||= Hash.new{ |k,v| k[v] = [] }
    if block_given?
      @sections[key] << block
    else
      @sections[key].inject(''){ |content, block| content << block.call(*args) } if @sections.keys.include?(key)
    end
  end
end

some_view.erb

<% section(:sidebar){ partial :custom_sidebar, :local1 => blah, :local2 => blah } %>