Touch probe woe's

[m_c]

That complicates things somewhat.
The diagram for the probe is just a basic functional diagram then, and there's a bit more going on in it.

Have you measured the output voltage on the probe pin?
It might work by simply connecting the probe output (and GND) to the G540, but there's a potential the two might conflict with each other.
I would think given the probe is designed to accept 5-24V and with a NO output, it's output circuit should handle the 12V from the G540.
My concern would be if the probe voltage is grossly higher or lower than the 12V, but it all depends on the exact circuit in the probe.

This is the problem when there's a lack of documentation.

If needed, I'd implement a basic NPN transistor setup.