I'm confused by some of the responses here.
The Ptolemies, and their ruling class, were a thin veneer over the native Egyptian culture. The locals stood for it, because they weren't organized and the Ptolemies crushed revolts.
If the Celts decapitated the Ptolemaic leadership, and appealed to the local Egyptians, I could see them holding out. If you were a native, would you rather have a foreign dynasty that considered you barbarians, or one that married into your leading families and started assimilating - and who desperately need your support?
It would be ... very, very tricky to pull off. But I imagine the other Diadochoi states might even give subtle support - get the Ptolemies out of the game for a while, and take over their conquests (in the Levant, Cyprus, wherever).
The only way they survive more than 1 generation or so, is if they are able to pull off some cultural fusion, marry into the local ruling class, encourage Hellenic studies and commerce, but not military, etc. As I say, tricky. But, IMO, not impossible.