It's not that hard to make a plugin to TinyMCE, similar to what the Wordpress gallery is doing in WP 3.5. Unfortunately, these custom placeholders are always non-resizable, although they show the resizer handles.
A new image inserted into a post is resizable with the handles, an embedded iframe is resizable, but a new gallery placeholder is not resizable (in this case it doesn't make sense, galleries don't resize). However, I need a custom placeholder that is resizable. Is there a special class that turns this on or off, or a function that I need to call to enable resizing on a certain element?