Maintainer of Rice here (https://github.com/jasonroelofs/rice). The problem set of trying to seamlessly integrate C++ and dynamic languages is not a simple or trivial thing to accomplish. I know Rice isn't particularly easy to use but it does have a lot of similarities to the OP's library and Boost.Python API wise.

I'm curious what you found gnarly or difficult to use, maybe there are some improvements that can be made?

Also as far as I know, Rice is the only library of its kind in the Ruby space (ignoring SWIG, of course).

The MRI C extension API is non-existent compared to Lua or CPython.