Chris, close but no cigar for you today
There's three classes of device driver for Windows CE: Native drivers, bus drivers
(no puns please
) and stream drivers.
Native drivers are created by the OEM to allow WinCE to talk to the hardware platform and take the form of touchscreen, keyboard or other low-level system device drivers. You do need Platform Builder to build those.
Bus drivers provide the interface to system buses
(PCI, SD, PCMCIA, CF etc
) - again, PB is needed in some instances depending on the architecture.
Stream drivers are standard DLLs with 10 entry points that *can* be build using EVC4. There's a good chapter introducing device driver writing in Doug Boling's "Programming Windows CE .net" book, and I'd recommend you hunt that down or search through MSDN for more information.
You should be able to write this with EVC4, however you might have fun getting the necessary information on the lower level of the stack, etc, that expects to use you and also you'll need information on the bus that you're using - again, you should use the existing driver for that though.
Good luck! Please keep us informed as to progress that you make as I'm fascinated by this, but won't have the chance to do what you're doing.