There is a practice circuit that includes outer high speed oval and inner ovals, 50m to 200m. ( I think )
Also a marked out braking area, were you can test your braking distance.
But no recording facility, cannot remember if MoTeC (acti) worked on this practice circuit.
If it does not you could use MoTeC to measure braking distance on Le-mans straight, tthen measure braking distance using MoTeC related data recording facility.
Look up acti in racedepartment's download section for assetto corsa
I forget the name of it but there is an app (it might be called acceltest) you can set the brake or acceleration test parameters in the .ini and use it to test stopping distance. The best way is to use the drag strip (accesible in practice mode only) and do a fair few runs as you can get some bad results sometimes. I've used it a few times to test mod cars against RL tests.