Actually, Jason, we did discuss this question here:
http://discourse.textkit.com/t/what-kind-of-genitives-are-these/11909/1
On that thread, I wrote:
I can’t see how these can be technically epexegetical genitives since “assuming a form, that is, a servant” makes no more sense in Greek than it does in English.
On a side note, Stephen Hughes recently suggested on B-Greek a new (new, at least, to me) way to understand genitives. You pretend that the genitive case does not exist, and you replace the genitive phrase with a relative clause in which you use a verb that is implicit in the relationship between the two nouns. Thus
ἐν μορφῇ θεοῦ
can be paraphrased as
ἐν μορφῇ ἣ ὁμοιάζει τῷ θεῷ
or
ἐν μορφῇ ἣν ὁ θεὸς πάρειχε.
In my opinion, this is a better approach than trying to categorize genitives with English meta-language terms. A true epexegetical genitive, i.e. Rom 4:11
καὶ σημεῖον ἔλαβεν περιτομῆς
would be paraphrased
καὶ ἔλαβεν σημεῖον ὅ ἐστιν ἡ περιτομή.