As I understand it, SMPTE is the industry standard for broadcast captions. Since most live streams are produced for broadcast (probably too general of a statement), SMPTE makes a lot of sense, as it doesn't require any extra encoding. For what it's worth, I believe Apple already support SMPTE for HLS.
Either way, here's the post with the text I quoted in my previous response:
viewtopic.php?f=34&t=51113&p=346903#p346903RokuPatrick may be able to give you more details, if necessary.
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