[Nonfractal]

Interesting thread Alan, no experience of Slot.It as I use a Truspeed MT11 controller which seems to work differently and possibly does what you need:

"A single rotary control sets the braking strength and has a built-in hold circuit (similar to the PWM controller) to release the brakes once the car has slowed to a speed relative to the amount of braking you have selected. The useful feature of this design is that the motor drives the brake circuit in such a way that the faster the car is going, the harder the brakes are applied. As the car slows, the brakes are gradually released to give very balanced and predictable control. The brake and hold remain balanced to leave you free to race confidently."

My reading of this is that you can run the brakes at less than 100% and still have the 'sweep' function but I may be wrong, how do you interpret it?

It had never occurred to me that the single brake control on the tru-speed controllers could achieve this function.
It appears that truspeed PWM controllers are always in an adaptive mode and they apply brakes depending on  the feedback from the motor.
I'm a bit confused regarding how the controller could achieve this, after all, there is no way that the controller can mitigate against the track surface and the tyres without sensors on the wheels (real anti lock braking) 
Once the tyres lose traction, the feedback from the motor speed isn't a measure of the speed of the car. 
Are stationary wheels evidence that the car had come to a halt or that the car is skidding? 

Likely that Steve Hills from TruSpeed  is the only person who could answer this question. 
Alan