Porting custom build steps from zzdoc to zig 0.17

I’ve been looking at updating GitHub - rockorager/zzdoc: an scdoc compatible manpage compiler for build.zig · GitHub to Zig 0.17, but it depends on being able to make a very custom build step that executes a zig function, which seems to be gone after the Configurer/Maker split. The question is, is this gone forever (probably because it was an impl-dependent hack all along), or is there some undocumented internal std API that can do roughly the same thing?