Got this figured out this morning. The root issue is that HTTPS redirect was enabled at the east site. By disabling this redirection, ProxyPass started working.
The follow-on change was using ProxyPass over HTTPS, which was achieved with the following configuration:
user@master:/etc/apache2/conf-enabled# cat multisite_proxy.conf SSLProxyEngine on SSLProxyVerify off SSLProxyCheckPeerCN off SSLProxyCheckPeerName off SSLProxyCheckPeerExpire off <Location /prod_east> ProxyPass https://east.domain.com/prod_east ProxyPassReverse https://east.domain.com/prod_east </Location>