How can I use custom classes in chapter titles with Asciidoc epub3 converter?

Question

In the adoc file I define a chapter header like:

== [big-number]#2064# Das Spiele-Labor

For HTML that translates to

<span class="big-number">2064</span>

For the epub-Version, converted with asciidoctor-epub, apparently the class is omitted. The code line in the converter.rb:

<h1 class="chapter-title">#{title_upper}#{subtitle ? %[ <small class="subtitle">#{subtitle_formatted_upper}</small>] : nil}</h1>

(/var/lib/gems/1.9.1/gems/asciidoctor-epub3-1.5.0.alpha.7.dev/lib/asciidoctor-epub3/converter.rb)

How can I get the class information over to the chapter-title to format the first number in a special way?

Or is there another way to solve this? (The first number of the chapter title should be large and CSS hasn't got a 'first-word' attribute)