It works!

French subtitles with diacritical marks in srt format (SubRip, as Western ISO-8859-1), and in ass format (Advanced Sub Station Alpha, as UTF-8) survived the conversion to Scenarist Closed Captioning (scc). Ligatures like œ and typographical quotes did not make the conversion, you might want to run a search and replace before converting with SRT Converter. I assume that is a limitation of scc.
The timing was out of sync in the Compressor 4 preview, but that seems to be a Compressor issue, the subtitles where in perfect sync in the finished compressor job.
There’s no documentation and no help, but the application is self explanatory: Load your subtitles, set the desired output format, click convert, that’s it.

. about SRT Converter, v1.0.1