As summary says.
In case of kde USE flag, no multilib-related changes will be needed for >=net-libs/libproxy-0.4.12, which calls kreadconfig5 / kreadconfig executables instead of using libQtCore.so.4 (dev-qt/qtcore:4) + libkdecore.so.5 (kde-base/kdelibs:4). (Bug #574046)
There hasn't been interest in doing this in 9 years, and libproxy-0.5.0 has dropped the spidermonkey and webkit pacrunner backends, so I'm going to mark this as WONTFIX.