OnStepX is very much pulse based as far as I understand it. I haven't encountered offloading techniques like you mentioned yet. Since both motors support (multiturn) positon modes, it should be possible to completely forego the pulsebased approach and only drive the motors using regular (and less frequent in comparison to pulses) position instructions over CANBUS. I spent some time reading the code in OnStepX to understand how to implement it in this way but hooking it up in pulsemode was just too easy to get started.