This is important for packages (like https://github.com/golang/glog) where the flags are embedded deeply into the code.
I already have this supported in kubernetes here that we can borrow/steal:
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/util/plog_import.go
(I typo'd the name of that file and I'll fix it up soon).
This would be a lot easier if the pflag.Value.Type didn't exist or was an extended optional interface. It is unclear why that is there and where it is used.