Your outdoor wood boiler is probably your best option at this point. Rocket stoves were not intended to do this type of work, but there are people who have proposed ideas and are working on better solutions, but I have not seen anything ready for prime time yet. If you've got the time to experiment, then I'd say go for it because we need more people working on this. If you really want to use woodchips, you might consider starting with the freely available FEMA Wood Gasifier Plans and seeing how you can use the resulting gas to heat water in a mostly off the shelf water heater. It would take some effort to design an automated feed and starting mechanism for the gasifier, but many of the commercial gasifier systems have this, but I don't think there are any open sourced versions out there.