Sharing Gems between Chef and System Ruby
To take your points one by one:


  but in the interests of good system hygiene I would like to avoid as much as possible duplication


Can you ensure your system ruby will evolve at the same pace as the Chef's ruby ? Main point of chef coming as a bundle with its own ruby comes back from Chef 10 which was installed via a simple gem command and was usually a problem switching between ruby needed for applications and ruby needed by chef, the same goes for gem dependencies. I would advise against mixing system and chef ruby.


  the Gems I have in the system Ruby come from the known-good local Yum repo and are installed via RPM over the LAN whereas chef_gem goes off to the public Internet


While I understand and agree with the worrying about recent pipy events, there's two things here:


Your system ruby install gems from yum repository because your system ruby's gem command has been tweaked for that, that's specific to your distribution. -
Chef's ruby use usual ruby method going to rubygems.org, as any ruby installed by rbenv or any other way than yum install would do.


If you really want to use a local repo you'll have to set up a gem server (alternatively, some artifact repositories can handle that also) and set-up the sources for gem using chef's gem sources in /opt/chef/embedded/bin.

If you're using a chef client above 13.0 your can use the rubygems_url in client.rb to tweak that:


rubygems_url

  
  The location to source rubygems. It can be set to a string or array of
  strings for URIs to set as rubygems sources. This allows individuals
  to setup an internal mirror of rubygems for “airgapped” environments.
  Default value: https://www.rubygems.org.
  Changed in Chef Client 13.0.


And finally:


  I have tried setting ENV['GEM_PATH'] in my recipe both before and
  inside a ruby_block but this appears to have no effect, I get a
  failure on require


Chef omnibus package hardcode some values specially to avoid interfering with the system ruby, I think the GEM_PATH environment variable is ignored when searching for gem you require within the recipe code. I don't think there's a workaround for this, and again you're heading to a compatibility problem on mid term as you'll have a drift between chef's ruby and system ruby.