We talked about this being an option when we were designing the H9, but if it behaved like this, how would one toggle between bypass and active? You could try to bypass how you activate the tuner, but some people have had no end of trouble doing a two button press (I guess it depends a lot on what shoes you're wearing…)
I actually argued for making the footswitches configurable, but the truth is the more I've gotten used to using the H9 the more I've realized the problems with what people suggest as alternate configurations.
Another one that comes up is people want long pressing into tap tempo mode to allow them to control the performance switch instead of the tempo, but the problem with this is that the performance switch itself sometimes is press and hold, which would boot you out of tap tempo mode.
Other people have suggested assigning the performance switch to the big, black wheel, but again a lot of people have trouble hitting this wheel with their foot (let alone pressing and holding it).
Is there any reason why you can't use an aux switch? They're cheap, and they don't have any of the aforementioned drawbacks.