[agl]

You can just glue a wire to the touch plate and connect it to ground when you want to simulate a touch.

[paxys]

I would personally not wire any amount of current to a USB peripheral connected to my MacBook.

[threadpiper]

Its like grounding it, same thing you do when you touch it. pretty safe no?

[paxys]

I guess, but it depends on what the other end of the wire is connected to.

[simias]

I guess a ~10kohm resistor in series couldn't hurt, should something go wrong. After all your finger is not exactly super conductive in the first place...

[unwind]

Switching a wire between mid-air and ground passes 0 current in both states which sounds like a pretty safe amount, though.