I did wonder what voltage the G540 pulled the input pin up to.

In that case I think you'd get away with only connecting two wires to the probe, assuming it is as simple as that circuit diagram shows for it (i.e. only a pull up resistor)
Assuming there is no fancy electronics in the probe itself (I doubt it), the only purpose of the 5-24V, is to pull the probe output high.

So if you connect the probe ground, and output in a similar way you connected your touch plate, it should work.

The only thing to be aware of, is what is the rated current of the probe output?
I'd hope it'll be good for at least 20mA, but I'd also just double check how much current the G540 takes to pull the input low.