I think you can accomplish this with the search/replace function. First select the toc.ncx file, then use the following settings in search/replace.
If the image didn't work, then use these settings in the search/replace panel.
Find: <text>(.*?)</text>
Function: Title-case text (ignore tags) [select this from the drop arrow]
Mode: Regex-Function --- Current File
That should change all of the labels in your TOC to Title-case.