Why not simply look up the extended attributes using the given name of the principal when you get it?
I don't think the creation of the principal used by GF is exposed to the user (like, say, in a realm implemenation). If that were the case, you could extend GFs with your own and then down cast it in your code. But I don't think the hook exists.
If it were me, I'd just look up the extended object with the principal name. That's at least portable.
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