The only way to get max. 500 watt is to use max. 24 volts. That limmits your wattage to legal values.
Changing max RPM is only a trick to get lower speeds, but you will still have an to powerfull ebike.
But using max. 24V would not produce just under 500 watts of power at 32km/h which wilson should be able to legally use.
According to the
performance data sheet, a Magic Pie on 24V only produces 308.88 watts of power @ 130rpm
(around 10mph or 16km/h with a 26" wheel). The maximum speed with a 26" wheel would be around 15mph
(~25km/h) which is 7km below the 32km/h legal limit that Wilson has to comply with.
Most legal power limits refer to the motor's maximum rated continuous power output rather than the maximum available power. Check your legislation to find out exactly what it refers to.
If your legislation states an actual maximum power output (not the maximum continuous rating) of the motor, you can set your "Battery drawn current (A)" setting to 18 Amps, which should limit the actual power output to approximately 500 watts @ 72% efficiency.
This means that under full load, the actual power being drawn from the battery will probably be nearer to 700 watts, but 28% of the power being consumed is basically generating unwanted heat within the motor and controller.
If you limit the motor's current to ensure the output does not exceed 500 watts and also limit the maximum speed so that the motor can only provide assistance below the maximum legal limit, then why should the battery voltage matter?
If a Cycle Analyst is used on a 100V 10kW motor to prevent the power and speed from exceeding the legal limits, should this be considered illegal?
It shouldn't matter how the power and speed are permanently governed, so long as the bike cannot exceed the stated legal limits it should be perfectly legal to use.
Motorcycle manufacturers are allowed to simply restrict the power output of some motors so they can be used by learner riders, so if restricting the power to place the vehicle into a lower power classification is acceptable
(and legal) for motorbikes, it should be just as acceptable
(and legal) to do the same thing with eBikes.
Alan P.S. The 250W EU legal limit may be acceptable on flat roads, but it is not sufficient for steep hills as it forces the rider to slow down considerably. Most riders tend to become very unstable at slower speeds and will often wobble considerably from side to side, and this makes them far more likely to struck from behind by other road users, or cause a major collision if the vehicle has to swerve into oncoming traffic in an attempt to avoid them.
One of the things that motorists hate most is being stuck behind a very slow (and often wobbly) cyclist struggling up a steep hill.
If they were going faster and keeping in a nice straight line, it would be much easier to overtake them safely. 
I have never been able to understand why it is perfectly acceptable for a pushbike to speed down steep hills at speeds in excess of 40-50mph without a motor, yet they have to be limited to such ridiculously slow speeds when going up the same steep hills with a motor.
It is much easier (and safer) to stop quickly while going up a steep hill than it is going down it. 
