Conditional cache_peer based on transparent/non transparent connection?

[Ed W <lists@xxxxxxxxxxxxxx>]

Hi, I have an upstream cache_peer which requires authentication.  My 
local network squid accepts explicit proxy clients, and does transparent 
redirection on everyone else.

Clearly I can't use the cache_peer for the transparently proxied 
clients, however, how could I use it only for explicit proxy clients? 
(ie those with a proxy set in their browser).  The cache_peer line uses 
login=PASSTHRU so we never get involved with the authentication on the 
local squid

Thoughts on how I could achieve this please (without two squid 
instances)? Squid 3.2.0.16

Thanks

Ed W