Harmy said:

I'm hoping that what Jan said will work (I will of course test it first), so if you already had the mkv, you wouldn't be downloading the whole BD but only the menus and extras (and yes, there would be some). I'm not sure how exactly it would work with sound and subtitle files though - I have no idea how the BD structure works.

Subtitles need to be in PGS (Presentation Graphic Stream) format for blu-ray.

When we drag your mkv onto tsMuxeR, the video, audio, chapters and subtitle streams should show up. But you may need to provide the DTS-HD MA track as a separate download if you're only going to include PCM, AC3 or core DTS audio tracks with the mkv.

You could also provide an even smaller version of the BDMV folder structure for those of us who also already have any of the extras you're going to be authoring with. In other words, the STREAM folder would only need to have the menu m2ts video files.