middleman_block_helpers.rb

helpers do
  def buffer()
    @_out_buf
  end
  def capture(buffer)
    pos = buffer.size
    yield
    buffer.slice!(pos..buffer.size)
  end
  def my_helper(&block)
    buffer << ERB.new(capture(buffer, &block))
  end
end

## Usage
# <% my_helper do %>
#  <p>This is the content in the block.</p>
# <% end %>