I've been working on something like this for myself.
Trying to keep it ALL plugin is proving difficult as from what I can see when the threadbit is processed only the standard thread table fields are available.
Adding a new field to store the thread tag is easy enough for editting but to retreive it when needed is not so simple
I've mocked something up just now that works by storing the thread tag in the title or notes field as they are available by default...
(not tidy & tested enough for sharing for a bit)
Hopefully someone else is having more success or better ideas?