Placeholder plugin for CKEditor v3?

In the original FCKEditor there was a plugin called "placeholder". It, pretty much, does what I need it to do for a custom plugin I need to develop (although it had some bugs).

Has anyone made a plugin like this before for the new version, or are planning on it?

Essentially we have special formatted tags (like BBCode, but singular - no closing tags) and we need to basically hide the syntax with a human readable label and allow the user to edit the tag via drop down in a CKEditor window. Like "[tag_name]" would show "Tag Name" in the editor and "Tag Name" is in the drop down with the value "tag_name".

Any ideas?