In the 3E book "Deities and Demigods", proxies are actually made into demipowers! Wouldn't this bar them from Sigil, unlike in 2nd Edition? Or shoud "proxies" be created using the presige classes presented in "manual of the Planes" and "Epic Level Handbook", perhaps?
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Some of the proxies in On Hallowed Ground were demipowers as well, especially in the Celtic pantheon, where "proxies" were often other aspects of the gods.
Yes, of course those proxies who are gods can't get into Sigil.
Your typical proxy is obviously not a god, and prestige classes would work fine to represent them. They're more like 3e's herald. A proxy doesn't actually need any template or class at all. Qaida, for example, wasn't given any additional powers for her proxyhood. In Planescape parlance, a proxy is simply a being who serves a god directly. No special rules are needed.