If the fan is only to remove moisture from the bathroom then you can vent it to the living space. If you want the fan for odor control, then exterior venting is the only way to handle that. MLB's soffit idea would work just fine for that.

Because you mention mold and moisture issues, it leads me to believe that the primary purpose of the fan is for moisture control. As I said, you can vent that directly to the living space without any issues. Another option would be to leave the door open and run a floor or desk fan to circulate the air after using the shower/tub which is creating the excess moisture problem.