> I appear to be able to access the parallel port using a separate piece of software that allows me to manually set various pins
> high/low, and I can verify that this is working using Inquisit's "receive" function in the port monitor - in other words, if I manually
> setpin 9 to "high" I am able to receive the code "128" in the Data Register (again, all within the same computer).
How do you verify that you are indeed physically accessing the parallel port using said software? It might just be emulating a port signal completely in software. In any case, if this indeed works as you think, you should be able to set a given pin to high on machine A and receive that signal on machine B.
The fact that the port monitor (in your within-machine scenario) shows a successful signal reception suggests that things are working fine on Inquisit's end.
Have you checked your port settings (LPT#, base address, etc. in the device manager) and verified you've set everything up correctly? Have you made sure the cable used to connect the machines is wired correctly (you'll probably want to find a LapLink-style crossover cable)?
> Are these port replicators incompatible with Inquisit?
I don't know anything about those replicators. It all depends on whether those Dell boxes expose a "true" parallel port, not merely some internal USB-to-parallel interface that's only sufficient to drive a legacy printer. Presumably Dell provides relevant technical specs somewhere online.
> Is there any other solution for modern systems?
Yes. Add-on cards (PCI, PCIe) are available that provide true parallel interfaces.