Orange peel is one of those iffy items for James -- it's less loaded with citric acid than oranges per se, but he'd still rather avoid it. Substituting for citrus is always difficult. There are so few foodstuffs with the same zing. Verjus, rice wine vinegar, cider vinegar, booze -- there are a range of possibilities but few with the tang of oranges.
Tamarind came to mind. At Trader Joe's we found an excellent dried (but soft) version seasoned with peppers that had just the spark we wanted. So we are happily sipping our tea, hoping to outlast the colds, and enjoying one more discovering in the world of culinary substitutions.