The AVR will act as the supervisor chip and it will be programmed with static Z80 machine code in an array (possibly in Flash using the PROGMEM variable modifier). Upon startup, the AVR will initialise it's ports for output and bring the Z80's /WAIT line low to suspend it's operation. The AVR will then load the SRAM, byte-by-byte from the array.

After this process is complete, the AVR will set the ports dealing with the data and address buses as inputs, therefore making them high impedance (suggested by Bill Rowe - Thanks). Then, the AVR will bring the Z80's /RESET line low to initiate reset, make the /WAIT line high and finally make the /RESET line high to start the Z80 running the program now in SRAM.

The challenge at the moment is to take each of the characters in the array and load them on to the data bus one bit at a time.