The TV Guardian people actually hold a patent on their method of monitoring closed-caption text as a trigger for when to mute the audio and substitute alternate text. So, Roku or anyone else wanting to implement this as a feature would probably have to pay to license that patent, and would need enough customer interest to make a business case for taking on that expense, along with development costs.
Unlike on Android and some other platforms, Roku apps are not (as far as I'm aware) allowed to run in the background and interact with other running apps, so a third-party add-on service is probably not possible.
Youtube channel "Technology Connections" recently posted an interesting video on how the standalone TV Guardian worked, after which "Ben Eater" did a follow-up focused on hacking one. The original device has to be inserted into a composite video signal path, though, and so is no good for modern HD equipment. With the prevalence of delayed or out-of-sync captioning text on many programs, I'd question the overall effectiveness of these things too.
