Plank Cap,
Sorry, I don't have any further info about the tests, other than what it says in the SB:
"Boeing analyzed the RTO events and performed simulator tests, and concluded that it is acceptable to continue the takeoff roll if a main gear steering actuator becomes unlocked at a speed between 80 knots and V1."
As a progression to this, would you choose to reject for the parking brake set master warning at let's say, five to ten knots below V1, assuming no other abnormal indications?
No, I wouldn't reject the take-off in that situation. At that speed, it's more than likely a spurious warning - If the park brake really was set, the warning should have happened as soon as the thrust levers were advanced for take-off. I would suggest the chance of there being a real problem is very remote and that there would be a much greater risk of rejecting the take-off at high speed and stuffing it up.