https://github.com/iuchim/zenhanのC実装版、ライセンスはオリジナルに準じる（public domain)

zenhan.c

#include <stdio.h>
#include <windows.h>

#define IME_CONTROL 0x283
#define IMC_GETOPENSTATUS 0x5
#define IMC_SETOPENSTATUS 0x6

int main(int argc, char** argv) {

  HWND fg = GetForegroundWindow();
  if(!fg) return 0;

  HWND ime = ImmGetDefaultIMEWnd(fg);
  if(!ime) return 0;

  LPARAM status;
  if(argc < 2) {
    status = SendMessage(ime, WM_IME_CONTROL, IMC_GETOPENSTATUS, 0);
  } else {
    status = (LPARAM)atol(argv[1]);
    SendMessage(ime, IME_CONTROL, IMC_SETOPENSTATUS, status);
  }

  printf("%lld", status);
}