Hi Tracker,
In general, no traditional medical qigong master is going to promote coffee drinking. When compared to tea, they might refer to the warmth and calming effect of tea versus the speeding up effect, or fire in coffee. In a qigong practice where you are trying to relax and open up, as well as slow down the mind and thinking, coffee doesn't really promote that. The ideal drink that sets sort of benchmark, is yin/yang water. On a physical level, it is half hot/half cold, so you are introducing a balance into the body. One can also infuse the water with a healing intension(spiritual level), transforming it into medicine. There are of course tea's that can be see as herbal too, or medicine in that sense.
Keep in mind that behavior and attitude is as important as what you consume. Sometimes a person can fixate on diet, and be ignorant of the workings of the mind and heart. If you drink coffee, don't beat yourself up. Just focus on your practice. It is often a source of change, and always a source of finding balance in your life.
love,
gallen