I've actually found it quite difficult to overheat the engine anywhere near the point of boil-over unless it's a very hot day and I've got the A/C on and the engine under load. At 1000-1500 rpm with no load on the engine the mech. fan seems to pull enough air to keep the temp high, but well under the critical point....at least on my car. YMMV. Obviously you should test it out first so that you're satisfied that you won't do any damage to your engine. I had replaced my water pump, hoses and coolant within the prior 6 months, so that probably helps as well. If it does start to get near the 120 mark, just shut off the engine for a moment, pop the hood and carefully reconnect the fan wires and then restart the engine to keep the coolant moving. You'll be able to see the temp start to come down within moments.

Experiment, use common sense and caution and I don't think you'll experience any dire consequences. But in the end, do no more than you're comfortable with.

Gary