why use gravity if you are just trying to heat water? hook it up to your main water supply and use that pressure to run water thru the RIMS. it would essentially be a tankless water heater. with the heater on full blast, just adjust the flow rate so that it comes out the desired temperature. that way, theres no need for a precarious heavy bucket above your head, and the flow rate would be consistant.
one thing you need to think about though is to make sure to cut power to the element when you turn off the water supply. if you ever forget, and if your valve is pre-RIMS tube, you will boil the standing water in the tube until the element breaks. if the valve is post-rims tube, you will heat the water until the weakest link explodes... at which point you will have a big leak around high voltage equipment to deal with.
edit:
to give you an idea about the flow rate you would get out of this...
assuming you were heating water from 60 degrees to 160 degrees (+100*), and using a 2kW heating element, you could heat about 7.8 gallons per hour to 160 degrees, or about 0.13 gallons per minute...