It is a hack, but as long as you are not sinking too much current you can use any GPIO pin on this chip as ground. Really.
How does it work? Every GPIO pin has a built in protection diode that connects it to ground...
Notice that if the voltage on the GPIO pin is lower than the internal ground bus on the chip, that diode will be forward biased and current will flow out the pin. As long as the total voltage between the external Vcc connection and the GPIO-to-ground connection is high enough that the voltage inside the chip is enough for it to operate, then it will run. Note that you have to account for the voltage drop across that diode (likely about 0.6V) and you can not exceed the rated current capacity of that diode (likely single digits of mA).
Wire up this circuit...
simulate this circuit – Schematic created using CircuitLab
...and then program the chip fuses to run at 1Mhz and write a tiny test program to set pin P2 to output mode and toggle it at, say, 1Hz and you should see blinkness.
- The power supply voltage must be high enough that the chip see a good internal voltage even with the drop across the protection 5V should be more than enough, especially with the chip running at 1Mhz where it only needs an internal voltage of 1.8V.
- The power supply voltage must be high enough to be bigger than the forward voltage gap across the LED including the the protection diode drop. Again 5V more than enough.
- The LED should be set up to SOURCE though the chip rather than sink into it. This limits the current flowing though the protect diode on the erstaz ground pin and instead uses the Vcc pin to drive the LED current.
NB: For the haters out there who say that connecting the LED directly to the GPIO will make the world explode, I say try it. If you are really worried about destroying humanity in the process, then connect the circuit to a variable supply and start with a low voltage and work your way up.