Well, I see what you are saying, Willy Nilly, and your motives are just fine. But as a host, I am paying the staff to take good care of you -- including remembering a special request for some guests, which really doesn't strike me as anything so out of the ordinary. It averages out anyway -- there will be some guests they don't have to serve at all, but I'm not going to subtract from the tip for that.
I can see if the bartender helped you carry your spouse out to the car, or if a waitress came into the ladies room to help you sew up a tear after a wardrobe malfunction, or the something really extraordinary like that. Maybe even if you requested a cocktail that required the bartender to run around the hotel during her break to find some bizarre ingredient. (I do not include in that a request to go and get a higher end liquor or more expensive wine than I as the host have provided. It's not polite for a guest to ask for upgrades whether or not they are willing to pay.)
But serving my guests what they want, the way they want it, even if it requires a little extra effort, is exactly what I am paying -- and tipping -- the staff for. If a guest tips extra for some special but not unreasonable request, that still feels like they are taking over my hosting responsibilities a bit, and I wouldn't like it. As a guest, in your position, I would simply repeat "Johnny Walker Black and soda water, not too strong, please" rather than pay the bartender to remember it. If a staffer does an exceptionally good job for you, the thing to do is to tell the hosts what a good job the staff did, so that they can increase the tip accordingly -- not to do it for them.
(Do professional bartenders really assume that "Scotch and soda" means Scotch and COKE?! Yeesh.)