We have many customers who run Inquisit experiments with EEG measures.
Inquisit supports signaling external devices via the parallel and serial ports. For a brief overview of how this works, check out the following link from our documentation: http://www.millisecond.com/help/v2/html/howtosendport.htm.
I scanned the Neuroscan web site for docs on the how to interact with their system, but they don't seem to have detailed manuals online. Inquisit is very flexible with respect to the timing and content of port signals, however, so you can present the right triggers to the EEG devices and synchronize those triggers with events in the Inquisit script (stimulus presentation, responses, etc.).