Your solution of having asm functions is possible but is not very efficient - why do an XOR? Something like:

mov a,#$4e
ret

mov a,#$55
ret

would do the same thing. You could also do the same in 'c' by using a switch and the compiler would most likely generate much the same assembler code. But then again. the compiler might generate a jump table and use movc to lookup! Can you not locate the movc disable to another area of the code memory? Or does it protect the whole lot?