if there are water lines near the furnace return.... use the window fan suggestion. and install a small hot water radiator. between the fan(s) and the furnace inlet air.. plumb the radiator from the hot water line to the cold water line. using a small hot water circ pump which should include a check valve. place the pump on the cool side of the radiator... that way the cooled hot water will return to the water heater via the cold water line.... use the water heater in electric mode, as it is a large gas consumer when used this way....
keep the fans on low and you have a constant heat source for the basement and living quarters... using portable electric heaters if it gets to cool for this system to keep you warm...
ps: you can mount extra radiators and fans in other locations like under the bathroom sink... just change the cabinet door panel to louvers or decorative screen. or mount in another convenient location... with access to water lines and power..