Currently we have:

java_protobuf_library(name='proto',
  sources = from_target(':proto-source-set'),
  imports = [],
  dependencies = [
   ...
  ],
  platform = '1.7',
  provides = artifact(org='com.squareup.external.protos',
                      name='foo-proto',
                      repo=square,),  # see squarepants/plugin/repo/register.py
)

so I'd either have to supply explicit support for all of these kwargs or just accept any kwarg and use it when generating the synthetic targt.