「所有者ウィンドウ」をブロックする解決策を見つけました。コードの最初の部分は Douglas の回答からのもので、残りは WinAPI EnableWindowメソッドの呼び出しを使用しています。
Window dialog = new MyDialog();
WindowInteropHelper wih = new WindowInteropHelper(dialog);
wih.Owner = ownerHwnd;
//Block input to the owner
Windows.EnableWindow(ownerHwnd, false);
EventHandler onClosed = null;
onClosed = (object sender, EventArgs e) =>
{
//Re-Enable the owner window once the dialog is closed
Windows.EnableWindow(ownerHwnd, true);
(sender as Window).closed -= onClosed;
};
dialog.Closed += onClosed;
dialog.WindowStartupLocation = WindowStartupLocation.CenterOwner;
dialog.ShowActivated = true;
dialog.Show();
//Import the EnableWindow method
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool EnableWindow(IntPtr hWnd, bool bEnable);