Here's a hypothetical plan:

1. Release movie as AVCHD

2. Release movie as mkv with a single soundtrack.

3. Release other soundtracks as synchronised individual files (maybe in multiple formats for each soundtrack).

4. Release extras as individual mkv files.

5. Enthusiasts mux their own soundtrack configuration to mkv with Mkvtoolnix (optionally convert to BD with ClownBD or experiment with basic menus with MultiAVCHD).

6a. Harmy works on his own BD and when happy releases all folders and files (except the video m2ts files) plus a template of conforming video m2ts files.

6b. Enthusiasts augment their mkv files, where required, to match Harmy's template, using Mkvtoolnix.

6c. Enthusiasts convert mkv files to m2ts with ClownBD and rename according to Harmy's template.

6d. Enthusiasts add m2ts files to Harmy's files and folders and voila, a BD complete with menus that should match Harmy's and without having to download a complete BD again.