Normally, an half-bridge topology is enough for both motoring and regeneration. H-bridge is necessary if you want to operate in both directions. For your application (bicycle), I guess only one direction of rotation is enough.
In order to regenerate energy, the average voltage supplied by your converter must be lower than the emf of the motor. In these conditions, the current flow will be reversed, and the motor will act as a generator.
You do not need to have a motor voltage higher than the battery voltage for regeneration. The regenerative DC to DC converter is there to supply the right voltage to the motor. If the supplied voltage is higher than the emf, you get power into the motor. If the supplied voltage is lower than the emf, you get power from the motor.
Remember that the emf is proportional to the speed. If you are running in one direction, the emf will always be positive. When the full battery voltage is applied to the motor, you are running at full speed. (Do not forget to put your helmet ;-)