The small space could be heated by running PEX piping between the floor joists, using the hot water that is on the way to the bath and using a return line back to the cold side of the water heater. The return line can be introduced to the cold line with a simple tee arraingment.
You may want to install a by-pass line for summer use so it doesn't battle the air conditioning. Also, any part of the pipe that is not heating the floor should be covered with closed foam (Armaflex) insulation.
You should be able to make an elaborate systme like this for under $100.