Alex,
It seems that the page is embedded in shdoclc.dll and there's no real way to redirect it to an externalized file. I could hack the dll resources, but then I'd have to keep fixing it anytime the dll updated.
>The
iis.inf
file contains references to all the error codes. You migh be able to work with it. It should be in the workstation even if IIS is not installed there (C:\Windows\Inf I believe).