React on cache clear event? Is there such an event?

I was looking for one of these as well. Use case: we have a caching NetScaler sitting in front of our Apache servers. It has a URL we can hit from the inside to clear the NetScaler cache. I was hoping to set up an action that would hit this URL whenever the Drupal cache is cleared, but I'm not having any luck finding a way to do this. Is it even possible? The only hook I can find related to cacheing is used to add tables that should be flushed.

Seems to me you could define a custom event to be fired from within hook_rebuild(). Then use https://www.drupal.org/project/rules_http_client to hit the URL. Or as long as you're implementing the hook, just hit the URL from within the hook using drupal_http_reqest() - it doesn't seem like there's any need to bring Rules into this.