After a little investigation, I think this is a path problem of some kind.
I tried two approaches to see what happened. First I opened a cmd prompt and tacked on my ghostscript path (c:\utils\gs\gs8.50\bin) to the %PATH% variable.
Later I tried adding the ghostscript bin folder in the system-wide environment variables under My Computer's properties.
I went on to try making the path really short. I cut it down to c:\windows\system;c:\windows;c:\utils\gs\gs8.50\bin Filemon still showed it never even attempted to read the ghostscript folder.
I tried adding a trailing slash to the folder name, no effect.
Finally I changed the order so that the ghostscript folder was in the middle of the path rather than the end, result SUCCESS. No more errors about EPS Input in extension-errors.log.
Can anyone explain this? Maybe a fluke with the compiler used and how it reads environment variables?
The plugin still doesn't WORK, but we're getting closer, anyway. I can see with filemon that it reads the .eps file I pick in the import dialog, but Process Explorer doesn't show gswin32, or any other child process, being spawned from the inkscape process.
I've attached my modified eps_input.inx too in case it matters.
Alan