In my environment, which is a K-12 public school of over 150,000 users, we implemented the custom categories and used the regex looking for the 'SafeSearch=Off'.
Then instead of blocking the user, we redirected them to an internal hosted custom web page that was similar to all the other custom EUN pages. This page we redirected them to basically has these instructions notifying the user they have been blocked because of safe search settings and how to change:
You have attempted to access Image Content without using Safe Search. For instructions to enable Google Safe Search, follow the steps below. (Note: Please read all the instructions first before continuing.)
Click http://www.google.com/preferences?hl=en
Scroll down to the ‘SafeSearch Filtering’ section
Choose the radio button for 'Use moderate filtering (Filter explicit images only - default behavior)'
Scroll to the end of the page, on the right hand side, click 'Save Preferences'
To view further information on Google Safe Search filtering, click here.
