ISAAC Perspective-n-Point implementation

Is the ISAAC ePnP implementation GPU accelerated or does it just use the CPU?

What would be involved in using this functionality outside of ISAAC, in another C++ application?

The ePnP implementation in Isaac SDK 2020.2 runs on CPU. Using the Isaac C API (here) might be a clean methods for integrating into another C++ application. Alternatively, you could try to extract the binary libraries from Isaac SDK and link them yourself, but this is unsupported.