back in the vanilla days, i had such an addon for my warrior... no longer can i recall the name of it.
she was prot so pvp was somewhat pointless besides stunning and annoying rogues going after our healers.
it would add a small timer (i think it was like 6 secs) to a frame and continuously countdown, until an action in combat reset it. you swing, get hit, cast, whatever and it reset. it was never hidden, always just bouncing back up to the max value.