I received my USB to parallel cable this week and I’m disappointed to report that the supplied driver does not expose an address space to the I/O registers. It is therefore not possible to directly control I/O through the port it provides. Indeed, it does emulate Centronics EPP and ECP modes, but it is doing so without a public address resource. After reading a bit more about a number of these cables, it seems clear that they are designed specifically to interface a parallel printer, and that’s about it.
Scrounging around the web I did come across a USB parallel port emulator project that looks interesting. It’s described in an article from Circuit Cellar magazine (issue 151 February 2003). The system is based on the Future Technology Devices International's FT8U245AM USB-to-parallel interface IC. I have a PDF of the article with a schematic if interested.
Andy