Cooling is just something that needs to be engineered in at the design phase, but it's no great shakes to get right. People are now beginning to use these RC model outrunners in small EVs and they are proving to be pretty good.
It a mistake to think that the propeller of a model aircraft provides effective cooling for these motors - it doesn't. In fact the prop actually impedes airflow at it's centre, as it tends to just throw air outwards at this point. The solution some big outrunners adopt is to have a second, concentric, cooling fan fitted behind the prop to draw air into the motor. Alternatively the engine cowl can be designed so that it has powerful extractor vents at the rear that do the same job.
Getting rid of heat efficiently is always a challenge, even with a bigger motor. The only real difference with these light RC motors is their low thermal mass, which just means they heat up more quickly. They do have some definite advantages though. Firstly they are often outrunners, so tend to have a higher natural airflow as the spinning can draws air through the motor. Secondly, they are made of light alloy, which conducts heat away from the core fairly well.
Given the experience of this chap: http://www.wisil.recumbents.com/wisil/shumaker/default.htm
it looks as if these motors can be run in lightweight EVs and still be kept cool. He's not drawing maximum power from his motor for long periods, but then these motors are so light that it would be possible to fit several, only using them all at full power for acceleration. Fitting a decent high pressure fan with good ducting to the motors would fairly easily deal with the heat problem.
They don't need to be as pricey as the Plettenberg motors either. These 6.5kW ones: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=5142
are only about Ã‚Â£75 each, which makes an array of them an interesting and affordable proposition.
I'm near-certain that these motors will be the basis for my next project - all I need to do is find some affordable HV controllers that are reliable enough. The Castle Creations ones seem to be the best, but are a bit expensive in comparison to the motor cost at the moment.