The PowerPC processor does not support operations directly on memory. You need to use a combination of loads and stores. You will need to over-write a register with the address that you want to load/store, and then use that register with the load/store. Make sure that the register you're over-writing is safe; when in doubt, r12 is pretty safe. But, for example, if the game needs an important pointer in r3, and you start over-writing r3, it will lose that important pointer and crash.
Here's an example that will do what I think you want to do.
lis r12,0x8079 # Load the upper 16 bits of r12 with 8079
ori r12,r12,0xDAA0 # Load the lower 16 bits of r12 with DAA0
lbz r12,0(r12) # Load the 8 bits at address 8079DAA0 into r12
stb r12,4(r3) # Store those 8 bits to address 4(r3)
Now, how to add this to the game? You would have to over-write four consecutive instructions to make this happen, but you probably only have one instruction that you can replace, the stb r0. This is a problem.
And the C2 code type is our problem solver. It will over-write the stb r0 with a single instruction, but that instruction will be a branch that goes to the four instructions above, and it will then branch back. Just make sure you add the instruction you replaced (the stb r0) to the C2 code (which we did, with the stb r12). It's an easy thing to forget and will end up causing you grief sooner or later.
By the way, use
PyiiASMH to convert ASM to C2 codes.
