Offload cached pages to different Apache process
It is not a direct answer to your question, but play around with setting MaxMemFree and MaxRequestsPerChild in the prefork section of your httpd.conf.  MaxMemFree should force Apache to free memory that it doesn't need, and MaxRequestsPerChild will force Apache to fork a new process (and free memory in the process) after servicing a particular number of requests.  You may have some performance impacts, but gain memory improvements.