CT Proxies and Forward Caches

Several content transformation proxies and the Internet in general
includes forward caches. Current definition of HTTP includes indication
of transformation using Vary header. In most cases the Content
Transformation proxies and servers vary their responses based on
User-Agent header. The number of User-Agent string in is very high and
caches cannot possibly store these mean copies of the response. Most
servers are likely to classify the devices in certain meta-groups for
the purpose of content transformation. However, this meta-group is
expected to be server specific. In absence of formal method, the caches
will be left to guess the meta-group. What will be the method to solve
this?