The purpose of such a probe would be to assist in troubleshooting, no? When one has a problem with an I/O pin the first step is to determine if the signal is getting to/from the I/O device. This helps determine if the fault lies in the I/O device (LPT/Mach in our case) or the external device (machine). So you have a 50/50 shot at the problem being on either side of the I/O device/external device connection.
What your proposing is using a pin on the very I/O device that you need to test to test the I/O device itself (the LPT). Sorry to say, you can't do that. Your testing apparatus has to be in a know good working state before you can rely on it to test anything. And, a testing apparatus cannot test itself!
If your probe was to find that a certain I/O pin was not acting properly how do you determine if the fault is the configuration of the port itself or some external device?