This should be achievable through a user script … I’m not good at scripting, but this should be easy.
Here is a user script which does the trick. Not pretty, but it works.
// ==UserScript==
// @name MusicBrainz UI: Make tracks collapsible in Release "Edit Relationships"
// @namespace http://tampermonkey.net/
// @version 0.1
// @description Make tracks collapsible in Release "Edit Relationships"
// @author You
// @grant none
// @include http*://*musicbrainz.org/release/*/edit-relationships
//// ==/UserScript==
/* global $ */
$(document).ready(function(){
$('#content>p:first-of-type').after('<p id="collapsibleEnabler"><a>Click here to enable collapsible media</a></p>');
$('#collapsibleEnabler').click(function(){
$('.subh td').each(function(){
$(this).css("width", $(this).width());
});
$('#tracklist tr:not(.subh)').slideToggle();
$('.subh td:first-child').append('<span class="collapse-mark collapsed"><a>▶</a></span>');
$('.subh').click(function(){
$(this).nextUntil('tr.subh').slideToggle();
$(this).children().find("span.collapse-mark").toggleClass('collapsed expanded');
$(this).children().find("span.collapsed a").replaceWith('<a>▶</a>');
$(this).children().find("span.expanded a").replaceWith('<a>▼</a>');
});
$('#collapsibleEnabler').hide();
});
});
[edited: managed to get the user script to work]
[edited: fixed the issue of column width of media row changing when collapsing the tracks]
[edited: fixed the issue of batch-add button disappearing, and added /▼ marks to indicate collapsible status]