driver.rb

require_relative 'escaped'

template = File.read('input.haml')
haml_engine = Haml::Engine.new(template)
output = haml_engine.render
puts output

escaped.rb

require 'haml'

module SafeMarkdown
  include Haml::Filters::Markdown
  include Haml::Filters::Base

  #this bit copied from Haml::Filters::Markdown
  lazy_require 'rdiscount', 'peg_markdown', 'maruku', 'bluecloth'

  # This bit copied from Haml::Filters::Base with only the addition
  # of the second parameter to the call to unescape_interpolation added
  def compile(compiler, text)
    resolve_lazy_requires
    filter = self
    compiler.instance_eval do
      if contains_interpolation?(text)
        return if options[:suppress_eval]

        text = unescape_interpolation(text, true).gsub(/(\\+)n/) do |s|
          escapes = $1.size
          next s if escapes % 2 == 0
          ("\\" * (escapes - 1)) + "\n"
        end
        # We need to add a newline at the beginning to get the
        # filter lines to line up (since the Haml filter contains
        # a line that doesn't show up in the source, namely the
        # filter name). Then we need to escape the trailing
        # newline so that the whole filter block doesn't take up
        # too many.
        text = "\n" + text.sub(/\n"\Z/, "\\n\"")
        push_script <<RUBY.rstrip, :escape_html => false
find_and_preserve(#{filter.inspect}.render_with_options(#{text}, _hamlout.options))
RUBY
        return
      end

      rendered = Haml::Helpers::find_and_preserve(filter.render_with_options(text, compiler.options), compiler.options[:preserve])

      if !options[:ugly]
        push_text(rendered.rstrip.gsub("\n", "\n#{'  ' * @output_tabs}"))
      else
        push_text(rendered.rstrip)
      end
    end
  end
end

input.haml

:safemarkdown
  Hello #{"<a href=\"evil\">My name</a>"}.
  <strong>But this remains</strong>.

out.html

<p>Hello &lt;a href=&quot;evil&quot;&gt;My name&lt;/a&gt;.
<strong>But this remains</strong>.</p>