How to/should I bypass switches?
I would put the plateau on 4 springs. One contact under the plateau, the other on the frame of the box. 3mm between the two contacts, aka "the switch". When the guitarist steps on, the springs get pressed and the contacts get closed. If the springs can be pressed with 1/10 of the weight of a person, the switch will be reliable IMO. 
It's a good idea to use a relay. It allows you connect whatever device you want. 
I would use 9 or 12V to power the coils of the relay. 5V is less reliable. Make sure the connection is either 100% off or 100% on mecanicaly and that contacts are never slightly touching each other.