transparent proxy forward authentication

I have the following problem:

For accessing some external systems, we have to use an http proxy. Then, I
configured a transparent proxy using privoxy so users can continue browsing
as usual but when accessing some pages, they are forwarded to privoxy and
it forwards the traffic through an external proxy.

All ok, but recently the external proxy was changed and now it needs
authentication.

I found this patch https://sourceforge.net/p/ijbswa/patches/134/ and
applied it myself. It worked ok, but I think something like this should be
implemented officially. I know it was rejected due to some limitations but
it there is a real need for authentication on forwarded proxy.