The documentation for ExecSyncOptions, used with execSync' includes the option appendStdio.
This misled me. When I'd been used execSync with NodeJS directly, I'd previously used 'stdio' to override the default [pipe, pipe, pipe] behavior to [inherit, inherit, inherit], because I wanted to see the child process's output in the parent. I'd assumed that the Purescript API would give me the same ability, but appending to Stdio doesn't allow me to do that.
Documentation of this difference from NodeJS wold be useful, or changing the API to allow for actually setting stdio.