Work on the Ruby interpreter is weirdly silo'ed off and mostly done by Japanese developers, so there's a significant barrier to entry for any enterprising C developer to roll her sleeves up and get hacking.
This is wrong. Ruby Developers welcome contribution in any form. Also, they have various resource to get started:
Official Contributing Guide: http://ruby-doc.org/core-2.1.1/doc/contributing_rdoc.html
Ruby Hacking Guide: http://ruby-hacking-guide.github.io/
Book on ruby internals: http://www.amazon.com/Ruby-Under-Microscope-Illustrated-Internals/dp/1593275277
RubySpecs: http://rubyspec.org/
Further, incase you are stuck. you can post on the mailing-lists. someone will surely help you get started.