The bus would be the stumbling block, the 601 to G3s used the 60x bus, and the G4 introduced the MPX bus -- but could also be used on the 60x bus. So there was some method to graft a fast chip onto a slooow bus.
The PPC 970 leaves these legacy buses behind, exactly what bus it is really hasn't been disclosed. But there is a good possibility that it'll need a bridge chip to work on the old bus.
Now, whether someone will make a chip is the question...
A bunch of upgrade companies could get together to do it, or a telecom could end up doing it.
Sort of comes down to, "Yes it can be done, but it'll only be done if somebody thinks they can make money or a market" -- this was the problem that nailed Apple when they included a new PDS slot in a specific machine that could be used for upgrades, and nobody made an upgrade for "that" machine -- and users demanded an upgrade via the courts.