> but it doesn't seem to be actively developed

The source code is here: https://github.com/rubyomr-preview/ruby

Seems the code is being developed internally to IBM. First the docker images of the OMR tech preview were dropped about a year ago. The the source was dropped recently. Targets Ruby 2.2 so I'm going to open an issue asking about targeting 2.3 and folding this work into trunk.

   $ git clone https://github.com/rubyomr-preview/ruby.git --branch ruby_2_2_omr --recursive 
   $ cd ruby
   $ autoconf
   $ ./configure SPEC=linux_x86-64 --with-omr-jit
   $ make
   $ make install

This should build it for you to try out. I urge others to try it out and give feedback.