Keynote Countdown Timer for Multiple presentations

I'm setting up a symposium consisting of six presentations of 20 minutes by various speakers. I'd like to incorporate a countdown timer that will show the time left in each talk in a box in the lower right corner. Ideally, the box would be green then turn yellow at 2 minutes then red at 0 minutes. As the moderator, I'd like to reset the timer at the beginning of each talk. My question is, can this be done as a macro that I would run repeatedly without having to make each speaker embed it in his/her presentation (i.e. can I run both my presiding macro and the speaker's presentation slides at the same time)? I'm pretty sure a timer can be constructed, but how can I work it with a minimum amount of involvement from the individual speakers?

Thanks anyone who has ideas. This seems to be a challenge!