Since I grew up with this type of heat in my bedroom (I remember scraping ice of the inside of the windows to see out), I will give you 3 options:
1. Install a small kicker fan at the gravity vent to force the air into the bedrooms.
2. Purchase a space heater for the bedrooms.
3. Run ductwork from the furnace to the bedrooms. If you are against tearing up a section of wall to accomplish this, you can run it outside of the wall (in an out-of-the-way spot) and build a chase around it to cover the ductwork.
Maybe someone else will come up with another idea.