I’ve been thinking more about initialization, and I think that it is sufficient to initialise A, and leave initialization of everything else to the first boot program written to RAM. Then write a second boot program (i.e. any 256-byte program) after that. The rationale is that:

• It doesn’t matter which 256-byte block of RAM the first boot program is written to, because the rest of RAM is filled with harmless instructions.
• Initialization of H,L and SP can be done with 3 single-byte instructions (assuming A is already initialised). So a repeating pattern of those 3 instructions + HALT can be the first boot program. 
• Because the 4 instructions are repeated again and again, it doesn’t matter where in the 256 byte block they are (so the initial value of L doesn’t matter) and it doesn’t matter what gets overwritten by the stack, so long as at least one instance of each instruction is executed. 

The sequence to initialise A is:

3E 00111110 LD A,n
2E 00101110
76 01110110 HALT
76 01110110 HALT

This sequence only needs to be executed once.

The programming sequence is:

3D 00111101 DEC A
2C 00101100 INC L
77 01110111 LD (HL),A
76 01110110 HALT

followed by this after NMI
(PC=0066h)

F7 11110111 RST 30h

The sequences only differ in how D1 and D0 are configured.

For the programming sequence the configuration is:

D7=A2
D6=A1
D5=1
D4=A1 or /A0
D3=/A1
D2=1
D1=A1
D0=/A0

For the initialisation sequence it is as above except D1=1, D0=0. So a DPDT can be used to switch between the two.

So the complete sequence of steps required to bootstrap the system will be:

• Disable /OE on RAM with a manual switch
• Ensure DPDT switch is set to ‘initialise’
• Start running host program on PC that sets up serial port (using fastest available baud rate) and sets RTS signal coming into the Z80 system to zero.
• Power on the Z80 system
• Wait for a second
• Switch the DPDT to ‘memory program’
• Tell the host program to program the first boot program.
• When programming has finished, reenable /OE.
• Tell the host program to send several NMIs to ensure that the 4 byte sequence of the first boot program is executed at least once.
• Disable /OE again
• Switch the DPDT to ‘memory program’
• Tell the host program to program the second boot program (i.e any 256 byte program)
• When programming has finished, reenable /OE.
• Tell the host program to send an NMI to start executing the second boot program.