I was wondering if an outrunner brushless motor with a belt to a larger gear would be effective
No, a single gearing would not be sufficient. You'd really need either multiple levels of gear reduction, a worm gear drive, or a different type of motor.
The motor you're looking at is specified with a nominal speed of 6000 RPM.
Most wheelchairs have wheels with a diameter of 24 inches (61 cm). To move at a reasonably fast speed of 5 mph (8 kph, 2.2 m/s), the wheels would need to rotate at ~70 RPM. (This is probably too fast for a wheelchair, but setting a lower top speed would make the analysis come out even worse.)
For a 6000 RPM motor to drive a wheel at 70 RPM, you'd need a gear ratio of roughly 85:1 -- e.g. if the large gear were the same size as the wheelchair's wheel, the small gear would be about a quarter of an inch (~7 mm) across. This is smaller than the drive shaft of the motor -- it's not clear that a gear (or pulley) this small would even be mechanically sound, let alone that it could transmit enough torque to drive a wheelchair.
Note that what I haven't analyzed here is whether this motor can even provide enough torque to move a wheelchair. This is because there are no torque specifications available for the motor you're looking at.