Sorry to respond to a four year old post but I’ve recently been dealing with a similar issue, having created captions at 23.98 for a feature film and finding the SCC out of sync on 29.97 encoded DVDs and for broadcast. So for the sake of anyone stumbling across this thread in the future…
24P caption files would work if the original source material is essentially 24 Non Drop Frame. Obviously if it’s 23.98/24 NDF that won’t help you. Furthermore, you’ve probably already found that even though the captions themselves are 23.98, since SCC files only really support 29.98, what you really want is captions set up for 23.98 but in a 29.98 wrapper.
The only program I know of that can convert any SCC files to work with either 23.98 or true 24p is MacCaption. True, it’s $1000, but most major broadcaster/production houses/distributors will have a copy, so you might ask them to make the conversion for you; it would take them about 40 seconds.