I had no problem muting a shared FX by assigning a soft key (or footswitch) but I can’t figure out if disabling an FX is possible for FX that are inserts on a specific channel. For example, the chorus FX on a rhythm guitar channel.
Unfortunately, as far as I know, it’s not possible to deactivate one of the sends in a shared FX – unless you set its FX send to infinity.
If you absolutely need that, you’d have to sacrifice a second FX unit with the chorus just for the rhythm guitar channel.
You could then deactivate it in one way like you wrote.
Odd enough, when using one FX in insert mode, you can still assign a soft key to mute that FX but it have no action.