UPDATE (some hours later):
1.) You don't need to issue any command-line commands to extract Blu-ray (BD) SUP subtracks from BD MKV files if you use the brother of the excellent (see THIS) MP4Tools, MKVTools. As with MP4Tools, you'll want to prefer downloading the beta from THIS page. For our purposes (subtrack extraction), it'll be just fine - the omissions / bugs in beta don't affect subtitle extracting.
All you need to do is as follows:
1, open the BD MKV file in MKVTools (“Open” button in the top right corner)