Hi Vickie
Personally, I would never, ever use any throat spray, or suggest the use of one to a student, unless it was prescribed by a voice professional. Eucalyptus oil doesn't actually do anything for bronchitis or laryngitis. It may make you feel you are doing something positive, but it actually aggravates the mucus membranes. Even natural products may have active ingredients which can cause aggravation which you can neither see nor feel. The vocal fold mucus is part of the structure of the vocal fold, and is of a specialised kind, being a delicate balance of gel and solids. Spraying your throat with anything foreign would probably upset that balance if it was to touch the vocal folds. Most of the spray is probably swallowed unless you actively inhale it into your airway. I wouldn't use a spray at all.
If your throat is dry at choir, it may be as a result of the way you are using your voice. Water will take approx 20 minutes from drinking to hydrating, and will only hydrate the folds after the vital organs are hydrated. If your mouth feels dry, just thinking of sucking a lemon can help start saliva flowing.
Best wishes
Frith