Currently enabling the interposer plugin only suceeds if either GSS_PROXY_USE=YES or GSS_PROXY_USE=1 is given, it should also work with "Yes,YES,yes,1,true,True" at least.
Patches are here: http://fedorapeople.org/cgit/gd/public_git/gss-proxy.git/log/?h=master-env
Fixed with v0.2.1
Metadata Update from @gd: - Issue assigned to gd - Issue set to the milestone: 2013 April