Ya, I've jammed an oversize flexible-ish France grocery store bought large bottle into a bottle cage after filling my official red MSR bottle, just to take it along, figuring if it leaked it would be better there than in a pannier.
FWIW, MSR says their regular fuel bottles will be damaged by putting alcohol in them. I have carried it in the bottles that bottled water came in. There is the danger that someone might drink it though. I drew big skull and cross bones on them. You could additionally ugly them up with duct tape or something. I tend to keep mine in my baggage so I am the only one I have to worry about drinking it. I find the lids secure enough that I don't worry about a spill. I do tend to put it with something other than my down sleeping bag though.
Actually 99% of the time on tour I just used yellow heet in the original bottles. At one point I did have a gallon of alcohol that I used for backpacking in bottled water bottles.I think I also used it for some tours that I drove to.