I dug into the PrusaSlicer 3 beta. This now includes a plugin API, sandboxed via Lua. It's fairly limited right now: you can create objects, emboss text, and insert custom G-code. The API is documented as comments in https://github.com/prusa3d/PrusaSlicer/blob/master/src/slic3r-shared/src/Slic3r/App/Lua/ProjectApi.cpp #prusa