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While I do not think it is possible/reasonable to mandate that all implementations support encrypted timed text tracks, I also think that it would be unwise to forbid or discourage CDMs from implementing that feature. As far as I can tell, the following Q&A is the only mention of timed text in EME -- the overall architecture appears independent of media type. Can't this (informative) text be simply removed, and the specification remain silent on the topic? "Q: Can I encrypt captions / <track> elements? A: No, this proposal only supports decrypting audio and video that are part of the media data." See recent thread at http://lists.w3.org/Archives/Public/public-html-media/2013Apr/0007.html
As discussed at the F2F, we don't believe that there is anything in the spec that prevents text tracks from being encrypted. In general, text track content is not consumed in a protected environment so encryption is less commonly needed. We will note in the resolution of Bug 21569 that text track APIs might not expose encrypted text track information. We should remove the FAQ about caption/text track not supported since in practice nothing prevents this. If anyone has a specific scenario that they believe is broken and a proposal for how to solve it, please file a new bug.
FAQ removed -> https://dvcs.w3.org/hg/html-media/rev/28b5fa4b9c59