Yeah just put an ASM branch in that general area, you usually want to leave the code immediately after the check intact, and you usually want to branch with a register that you load something into. EG. The routine has LDRB r0, [r2]. You'd branch using register 0, because its going to be overwritten by r2 anyway. From there you just add your own new checks, code etc. Make sure to return to the right places after though.
Best of luck!
Best of luck!