How can I grab an HTTP header on CACHE_REQUEST?

Yes, that logs the size for me. For example:

Feb 14 08:16:45 tmm tmm[6896]: 01220002:6: Rule dontcache : local0.3624

for a file of 3624 bytes.

I notice that if the object is still in the cache when I apply the dontcache rule, it still gets served from cache, but with a large Age: header. I need to clear the cache before that rule works.