Ray, My dad made a device to do that by stripping a couple feet of nicrome wire off of an electric heater and wrapping it tightly around the bottle to geti it hot quickly, then as you say dropping it into hot water.
The resulting cut edge was usually quite sharp. It might be interesting to try it with regular wire and a 12 volt battery but I wonder if the heat rise in the wire wouldn't be so fast as to burn the wire before transferring much heat to the glass. Tom Fowle
