This is a great little plugin. Solves a very specific problem very easily. Thanks!
I'm curious though why you don't have the excerpt editor in a metabox (with
<div class="postbox">...</div>. Even if you have it in a metabox, it won't be draggable (it needs specific classes and containers to make that possible) so you can still have the nice metabox style and not worry about breaking TinyMCE.
I forked the plugin for an internal project and tweaked it to make it work like this; here are my changes if you wanted to add them back to mainline:
print('<div class="postbox"><h3><label for="excerpt">Excerpt</label></h3><div style="padding: 10px 10px 13px;">');
The changes are pretty basic; just tweaked the container to have
class="postbox" and added a div wrapping around only the editor so it didn't bump up against the borders of the metabox.
I tested this pretty extensively in 3.5 trunk today and it works beautifully. Try as I might to drag other metaboxes around it, it all works beautifully. The metabox for the excerpt has a "move" style cursor on hover, but it's useless. It can't be opened or closed either.
Thanks again for a wonderful little plugin that saved me a bit of time and energy!
EDIT: I just saw you have this up on BitBucket. I'll go file a pull request there!