Grounding the pin does in fact mean connecting it to the battery negative/ground etc. I am not sure how well getting into boot mode will work with the ECU in the car as the car is always supplying some power to the ECU.
Do you have a picture of your ECU PCB? I know with the ME7.1.1 boards I am using a jumper must be removed from the bottom of the board before boot mode can be enabled.
You can easily get into boot mode with the ECU in the car. On my "development" ECU I made a pigtail which plugs into a short section of wire with a toggle switch that I ran behind the fuse panel. My car has a big EFK, so I've had a few flashes puke on the side of the road. Anyway, here is what it looks like:
I would not recommend this for daily driving... but you get the point. As Nyet said, the ignition is what boots the ECU, so I flip the switch to close the circuit, turn the key, then turn the switch off after 3-4 seconds.