Celtx has an Index Card feature. It is accessed through the Index Card sub-tab at the bottom of the editor. Index Cards are available in all editors.
You can use the toggle Show Script/Show Index Card that is located in the Index Card toolbar to make notes on the "back" of each card and to write Alternative Scene Headings for each.
Here is the Scene Navigator showing the actual Scene Headings.
Clicking on the Scene Nav toggle will show you the Alternative Scene Headings.