Despite setting CMAKE_CXX_COMPILER to $pathtodevtoolset-2/g++, the configure script still erroneously fills in the cmake files.
Specifically,
CMAKE_CXX_COMPILER:FILEPATH=$pathtog++
I had to manually edit CMakeCache.txt and CMakeCXXCompiler.cmake as nothing supplied to configure as exported environment variables would get this to build properly.
After doing this (and this is the wrong way to “fix” this problem), the plugin built correctly and inserted itself into bro.