What does the BMS documentation say about using the battery while it is being charged?
I am guessing that its okay, the BMS limits its own current. Then the 100A comes 85 from the battery and 15 from the BMS. Which should not be a problem.
There must be something I'm missing, as the charge circuits and the load circuits are usually independent, and one can charge the battery while it provided power to a load.
When you write bypass, so you mean disconnect?
If you need to disconnect the recharge when the battery is in use, look for a control input to the BMS that will do this for you.
Otherwise, put the MOSFET switch in the charging system low voltage return. Turn OFF the MOSFET when the current out of the battery is high enough.
You can put the MOSFET in the high voltage side, but it may be easier and safer to do it on the low voltage side.