Fun question!
My back of envelope calc estimate for top possible speed is about 1500 km/hr.
I assume the rider can put out about 200 watts. I also assume 0.9 coefficient of static friction of tires and road. I also estimated rolling resistance based on 40 watts at 50 km/hour on Earth scaling linearly to high speed (200 watts at 250 km/hr) but reduced by the lower gravitational force (so top speed is 250 x 6). And that would be the dominant force to work against since there is no gravity term and no air resistance term.
The rider would be limited by available traction to accelerate at about 6 km/hr per second so it would take about five minutes to reach that speed (and at least that long to slow down before turning around!).
However, thermal failure of drivetrain components might be an issue at anything close to that speed. Also the road has to be super smooth and the wheels need to be true and round.
Otto