LDAI value : ACC < RAM[RAM[value]]
and
STAI value : RAM[RAM[value]] < ACC
I've also added special instructions for writing to the "ROM". These will be needed for the bootloader, and changes my system's architecture from Harvard to Modified Harvard. Of course, this wouldn't have been necessary if I'd started with a von Neumann architecture instead.
The "ROM" writing commands use a dedicated indirect register, and there are commands for loading it (LDX), incrementing it (INX) and storing the Accumulator at where it points (STX).
It's not an elegant solution, but will do for now. Time to code a bootloader.