Small 3-5 gallon electric tanks are availble that fit in many sink-cabinets. If fed by the water from the main HW heating system, it's mainly only running in standby mode (about a 35-50W continuous load.) The volume of water in the pipe from the main hot water heater determines how much volume you need to counteract the dilution. (It takes 90' of 1/2" or 45' of 3/4" copper pipe to add up to a gallon. Min size on the tank would be ~ 3x the water volume of the distribution plumbing.)
The more expensive solution to install would be an electric tankless in series, but that would involve running a dedicated high current circuit- maybe even a new breaker panel. A tankless throttles back on power as the incoming water temp rises, but you'd still be looking at 10,000W or more to support any reasonble flow before the slug of hot water arrived from the tank.
In any configuration insulating the near-tank(s) plumbing and the distribution run from the main tank to the remote faucet location with 5/8" or 3/4" walled closed cell foam pipe insulation reduces standby, and extends the time between draws to where it's still hot enough to be useful up to about a half-hour. See: http://www.leaningpinesoftware.com/h...er_pipes.shtml