How to override expires, maxage, s-maxage on reverse proxy?

[Ricardo Newbery <lists@xxxxxxxxxxx>]

I've got a reverse proxy configuration in front of a content  
management system (Plone CMS if it matters).  Since the CMS can be  
configured to automatically purge content from the proxy whenever  
most content items are edited, I would like to force the proxy to  
retain the cache for a very long time without refreshing or  
revalidation.  In other words, I want my proxy cache to trust that it  
always has the latest version.  But I'm not sure I should do this via  
s-maxage in the response as this setting might also apply to other  
proxies upstream of me.

I looked at refresh_pattern and it's override-expire and ignore-no- 
cache options but I'm not sure I can leverage this.   First, because  
it isn't clear whether these options will override max-age and s- 
maxage if they're present in the response.  And second, because I  
need this behavior to be selectively applied (probably via a response  
header set by the CMS) since not *all* items on the CMS will  
automatically generate purge requests and there isn't a good way to  
filter these by path.

I guess what I'm looking for is something like an s-maxage that will  
be applied *only* to the reverse proxy and not to anything else  
upstream.

Any suggestions?

Ric