Why does it need to be a pack, why not handling the midi files like newgrfs?
You wouldn't be able to just download and install individual songs as with a NewGRF. There must be a "base pack" that OpenTTD loads before a game. For example right now you can choose to load either the original TTD graphics or the OpenGFX graphics, and likewise you can choose to load either the original TTD sounds or the OpenSFX sounds. They are loaded as an entire pack, although in this case there is no requirement for a minimum number of files required (although I can't say if there is a maximum, I haven't tested it that far yet). But there has to be a data file which links all of the music files being used to a human-readable name and there must be a MD5 checksum created for all of the included music.
So someone can upload his midi file to bananas and others can download the midi files they like, it would be much easier for the license stuff and also wouldn't need a need of discussion, which song should go into the replacment pack, which not.
The goal I'm trying to focus on here is to encourage the creation and approval of MIDI files specifically to be distributed with the game, so that it is closer to being a complete clone of the original TTD. Part of the TTD/OTTD experience is the excellent music that inspires gameplay and keeps the player engaged.
That said, it is still possible for somebody to develop an independently distributed collection on their own for users to download without all the discussion, licensing etc. All the user has to do is install all the files in the /gm directory themselves, and then select which pack they want to use when they start OTTD. I actually have my own ideas of putting together a replacement music pack of certain themes (holiday songs, ABBA's Greatest Hits, etc.) for users to download themselves, but as I said at the moment I'm trying to focus on getting a base set of MIDI files distributed with the game itself.
As far as downloading/installing packs via BaNaNaS, I'm not sure if the current OTTD 1.0.0 beta 1 can do this. As far as I know the files MUST be installed in the /gm directory where the OTTD installation is located, and not in the global file area. If it requires a re-write or inclusion of new code to accomplish that part, then I honestly don't want to hold this project up while we wait for somebody to find time to do that, if anybody is even interested in doing that. It's taken long enough just to get the OpenSFX project to a point where it was considered done enough to be released as part of installation and to get everybody to work on sound samples and such.