I'm afraid I have no good reason for my choice of propagatedBuildInputs/nativeBuildInputs/buildInputs - In pure cargo-cult fashion I looked at a few other packages' nix-expressions and imitated them. I'll have a closer look at the nixpkgs manual and see if I can make a more reasonable choice.

About dealing with video (I don't think that subtitleeditor actually edits any video) I haven't been able to play any of the x264 matroska files I used for testing. Until now I just assumed that maybe there was just no matroska support in gstreamer, but given your comment about referencing plugins, I'm not so sure. Do you know some application (off the top of your head) that references gst plugins by executable wrapping?