We can sometimes label sites as hacked in the search results, but Webmaster Tools is definitely the preferred channel for communication of stuff like this.
Ironic since the original reason for the tech contact was exactly to be notified for issues like this.
Would like to add that as a registrar though we get a reasonably good response to emails that we send. [1]
Perhaps there is something about getting an email from google that says "it's probably spam". Or any large well known company that is often the subject of spam attempts.
Even given that though it's hard for me to believe that the results were close to zero.
[1] We also find that when a domain is deleted for non payment the person frequently claims they received no email notice but then proceeds to make some reference to something that was on the email notice.
For example, if we manually detect that a subdirectory of a site is hacked, we may remove only that subdirectory from our search results. If your entire site is hacked, then your entire site may be demoted or removed until the site is clean.