Understanding ECU Lowside Output Circuit
What is stopping the lowside load just grounding via the 110nF
Capacitor when the IRLR3636 MOSFET is off?

Nothing. The load will still be 'grounded' - until the capacitor charges up through the load. If the load has low resistance then it won't take long for the capacitor to charge.
If there is no load then the capacitor will only charge up (slowly, through the 300k resistor) to the voltage at the junction of the 430k and 47k resistors, which is ~0.5 V. Therefore you will need to put a resistor in place of the load to pull the output up to the supply voltage.
The 'dummy load' resistance required depends on how much drive current your 'MOSFET/Relay/Driver' needs. A 1k Ω resistor would supply up to 1 mA with voltage drop of 1 V or less.
Remembering that voltage goes down when the ECU output is turned on, you may need an inverter to turn on your MOSFET at the correct time. This could be done with a bipolar transistor like this:-



simulate this circuit &ndash; Schematic created using CircuitLab