You can certainly use a pointer to a function for this purpose, but I'm a little unsure as to why it's necessary here. I'd expect all you're really doing is specifying ingredients and amounts, so you just need to pass that data, which might well be just an index into an array of structs held in progmem. Even if there are special handling instructions like "stir" or "add ice", these can be specified in the data as flags.
Finally, if there really are drinks that do need a function to handle them, you might consider specifying them in the struct. Most of these function pointers would be null, but it would enable you to override the default handling for drinks that need something unusual. Disclaimer: I haven't done this on an Arduino so I'm not sure of the subtleties that the Harvard architecture brings here - it would likely take a bit of time figuring out with progmem directives.
I'm looking forward to seeing the barbot in action.