Could an Overpower have an Avatar? If so, wouldn't that Avatar also be Divine, maybe the equivalent of a Greater Power? I know that Overpowers don't grant spells and rarely if ever make an appearance... but maybe their Avatars do! In fact, maybe one or more supposedly "normal" Deities are actually Avatars of an Overpower in disguise! This brings up another issue... why can't worshipers of an Overpower get spells based on their faith alone, the same way worshipers of a philosophy do? I know that in the Forgotten Realms, you HAVE to choose a Power, but this isn't true in other cosmologies...
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If you wanted, I guess so. You could argue that an overpower isn't just a more powerful power, but a different category of deity entirely. The personification of a crystal sphere, rather than a super-god. They might not have the ability or the desire to create mini-me versions of themselves, or if they did, they might be nothing like the gods.
On the other hand, sure - if you wanted, I don't see any reason you couldn't say, for example, that Boccob was an avatar of Greyhawk's unknown overpower.
Generally speaking, you have to know something exists if you're going to worship it. Outside the Forgotten Realms, overpowers are pretty much unknown to mortals. And most people don't even know about Ao.
An exception is Krynn's Highfather on the continent of Taladas, where the natives there revere the Highgod as a kind of unpersonified universal force. I suppose clerics of the Highfather in Taladas could get spells, if there were any clerics of the Highfather. I don't think there are - there aren't any clerics of Paladine or Majere there, either, since they see those gods as abstract forces as well. In Ansalon, they mostly don't know about the Highgod at all.