It may be possible. It appears that the pads are capacitively coupled to the chassis, likely for filtering ac noise. GND1 couples the HV input after the EMI filter and is not needed for testing; GND2 couples the output 12V low side return thru some large capacitors; GND3 couples the LV section gnd plane to chassis thru a single small capacitor; GND4 couples the L201 inductor 12V output return to chassis before the current sensor.
If you wanted to just troubleshoot the LV section of the board without trying to start and run the PWM, then only GND2 would need to be connected to the 12V output return. But there are so many necessary conditions in the logic gates that i'm not sure how useful it would be. The main 12V output power return occurs at the center tap of the T103 transformer secondary, so that is where you need to be careful not to run the PWM without full continuity to the chassis ground for the entire board/system.
But if you just wanted to power up the low side supplies to check Vcc at the chips, etc. then that might be okay without the full-blown GNDs connected.
For example, 15V could be put onto the TP201 pad to measure if the 5V reference is being created by IC209. i would have the meter connected to measure before briefly touching the power supply lead to the TP201. Just a brief touch to see the result and then remove it. And have the 15V supply current very limited to just a few mA.
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