Allow for custom vendor parameters (filters, map files, etc.) to be included in WMS and Cache requests.
For example, say I have a WMS service that takes additional filters e.g. "http://foo.com/myservice?REQUEST=GetMap&...MY_CUSTOM_FILTER=a_filter_definition"
If my cache service receives a request that includes the parameter 'MY_CUSTOM_FILTER', I would like it to include the parameter in the request to the source WMS.
GeoWebCache claims to support this: http://geowebcache.org/docs/current/quickstart/index.html
MapCache also claims to support this: http://www.mapserver.org/trunk/mapcache/index.html#features