I know you'd like this to be an included feature, but unless it gets done you might have to tinker directly with the source.
I fooled around with the code for a few minutes and here's what might work for you. I make no promises on it so please keep in mind that this is done at your own risk. I tested it real quick on my board and it worked for me, but that doesn't guarantee anything.
You'll need to edit the file located in /path/to/forums/admin/applications/forums/sources/classes/post/classPost.php
On the following lines, simply add // to the beginning of the line (two forward slashes, nothing else)
Line 1288 look for: throw new Exception( 'NO_CONTENT' );
Line 1316 look for: $this->registry->getClass('output')->showError( 'post_too_short', 103143 );
Line 1650 look for: throw new Exception("NO_CONTENT");
Adding the two forward slashes, the lines should then become:
Line 1288: //throw new Exception( 'NO_CONTENT' );
Line 1316: //$this->registry->getClass('output')->showError( 'post_too_short', 103143 );
Line 1650: //throw new Exception("NO_CONTENT");
That all simply prevents it from generating an error when the post is empty. It might also disable other important checks, but as near as I can tell, that will allow you to make empty posts. The drawback to doing this edit is that when a new release comes out, you'll have to do the edits again. But unless/until it becomes optional, this may be the only way to allow you to have empty posts.