Virtual parallel port

Hi,
Maybe you don't need to know how to write drivers. If I understand well you need somthing to simulate device. Have you tried adding another layer of abstraction?
Add an interface:

interface IParallelPort
 {
     void Output(int address, int value);
     int Input(int address);
 }

When device is plugged implement it the normal way

class RealPort : IParallelPort
{
    [DllImport("inpout32.dll", EntryPoint = "Out32")]
    private static extern void Out32(int address, int value);
    [DllImport("inpout32.dll", EntryPoint = "Inp32")]
    private static extern int Inp32(int address);
    void IParallelPort.Output(int address, int value)
    {
        Out32(address, value);
    }
    int IParallelPort.Input(int address)
    {
        return Inp32(address);
    }
}

For testing use fake class:

class FakePort : IParallelPort
 {
     void IParallelPort.Output(int address, int value)
     {            //simulate output
     }
     int IParallelPort.Input(int address)
     {            //simulate input
     }
 }

Use factory method to switch between those two ports.
Hope it helps.