Hi,
I don't think Mach3 is going to offer you a solution.
Machs trajectory planner outputs PVT ( Position, Velocity over Time) data in 1 ms time slices. To calculate the trajectory it
assumes machine has a fixed maximum acceleration, and after the acceleration period a fixed speed, be it a feed rate or a max rapid
traverse rate followed by a deceleration to its final end point. This is called a second order planner, that is to say the second differential
(calculus) is constant.
There are third order trajectory planners, that is third order differential is constant sometimes called 'constant jerk planners'.
Even such planners do not allow for varying maximum accelerations.
The motion you are contemplating is an example of kinematic motion where the trajectory is the solution of a differential equation
involving position, velocity an acceleration.
To my knowledge no CNC solution is arranged that way.
You might like to investigate various hexapod machines (also called hexapod robots) as they have a kinematic motion planner,
maybe you could adapt the idea to your machine.
Craig