Sounds good in general. Having the info in the same message instead of before every message seems to make more sense. Especially with how it is currently where you don’t really know what you get from the jtv user, whether it’s a command or an actual message to be displayed (except that the first word is all uppercase). An actual well-defined protocol sounds more stable.
How are you going to handle joins/parts/userlist which probably doesn’t get send to the webclient but may be wanted by others? Also you have “mod” in the example, will the +o modes still be there then? Not that special clients really need them (they can use the tags), but regular IRC clients might. (You probably don’t necessarily know at this time, but just bringing it up.)