Lemongrass is an extremely volatile oil, so it wouldn't take much to make the water smell. The question is, how much do you need to make the smell last, and how long do you need the water to smell? It could be that if the bees are retrained to that water source, it won't matter eventually. I don't really know about the peppermint either, but I'd want to be sure that their fountain wouldn't be damaged by putting oil in the water.
In my experience bees prefer dirtier water, so if you are using a 5 gal. bucket, don't change the water frequently, as the grosser it is, the more the bees like it. Also be sure to put something in the water so the bees don't drown in the bucket (packing peanuts, sticks, big rocks, etc.).