I don't think anyone has done anything with these and, yes, you're right on all counts as far as I'm concerned.
Something I did sneak in awhile back is another acs-templating/acs-integration proc called "ad_return_exception_template". It's useful for returning templated error pages, originally in my case those returned by on_error blocks attached to db_transaction calls. My idea was to start chasing down common error pages and replace them with templated versions.
Returning the standard pages you're talking about is easier because no parameters are involved (in the db error case the error message needs to be passed as a template param, in the general case arbitrary params and ad_return_exception_template supports that plus an error return code).
You might talk to Bruno Mattarollo about 404s. He implemented a templated 404 page for Greenpeace that fits into our scheme that allows arbitrary Greenpeace National suborganizations to use their own custom version.
He ran into some issues/bugs, in fact possibly the same one you're talking about or something specific to our stuff, I'm not sure (GP Planet runs a customized version of parts of the request processor). His email address is Bruno Mattarollo <mailto:bruno.mattarollo@ams.greenpeace.org> and he'll be more than glad to share what he's learned, if you want to ask.
