in the aolserver config file you can specify:
ns_section ns/server/${server}/redirects
ns_param 404 global/file-not-found.html
ns_param 403 global/forbidden.html
when using openacs you should probably use:
ns_section ns/server/${server}
.... cut ....
ns_param NotFoundResponse /global/file-not-found.html
ns_param ServerBusyResponse /global/busy.html
ns_param ServerInternalErrorResponse /global/error.html
ns_param ForbiddenResponse /global/forbidden.html
ns_param UnauthorizedResponse /global/unauthorized.html