Is there any mechanic in place for creating expandable/collapsable controls in admin screens? Eg, when showing a list of script log events there isn't room to show multi-line messages, and i'd like to hook up a click on on item to expand it to show additional information.

It is possible in Javascript with the following function:

<script>
$(".spoiler .toggle").click(function(){
    $(this).siblings(".content").slideToggle();
    if ($(this).text() == "+| Show") {
        $(this).text("-| Hide");
    } else {
        $(this).text("+| Show");
    }
});
</script>

 and the html object would have

<div class="spoiler">
    <div class="toggle">+| Show</div>
    <div class="content" style="display:none"><hr />
        CONTENT GOES HERE
    </div>
</div>

 I haven't tested this myself with the handlebars, but it might put you on the right track.  It works fine in standard HTML5