OK. I understand but even this tends to vary from room to room.
You have 3 ways of achieving the required result already that don't need to lock the Amp EQ section.
1- if you are using a decent PA (and sound engineer) then the best way to deal with making a guitar sit in the mix is to leave FOH EQ to the desk. If this in't an option.....
2 - Insert a Studio EQ or GEQ in one of the FX Slots and lock this. Downside is that you tie up an FX slot which you might want to use for something else. In which case....
3 - Go in to the output menu and tweak the Global EQ for the Main Output. As this is global it is automatically locked so you don't need to lock it manually.