Hi Simon,
Issue 1179 is the issue in OpenColorIO GitHub concerning the problem on which some help will be greatly appreciated.

In the meantime I only see a manual compilation as a solution to the problem i.e. compiling and installing OpenColorIO so that OpenImageIO can find it.
Please refer to the following file to understand how OpenImageIO is working to find OpenColorIO:
https://github.com/OpenImageIO/oiio/blob/master/src/cmake/modules/FindOpenColorIO.cmake

Patrick