See my comment about license silliness. I think what you're missing is that the GCC maintainers and FSF don't need to "accept" anything. I think this is going to be a distribution/user-driven thing, not unlink egcs or x.org.
Dragonegg is a no-brainer. The gcc plugin infrastructure means they can't stop it. Much compiler research is rapidly shifting to the LLVM world. Dragonegg is the perfect tool for this, meaning that at least in some areas, gcc will be playing catch-up in a few years. gcc has a big advantage in the number of targets it supports. Dragonegg leverages that.
The frontend is trickier but if the gcc devs refuse to allow it to be integrated (alongside the existing frontend initially) then I think we may see a mass shift toward Clang/LLVM. User experience is very important and Joseph Myers is missing something big. With the advent of many-core and massive threading via GPUs and the like, the compiler landscape is rapidly shifting. We need much smarter, friendlier compilers. The smarts are the relatively easy part. User friendliness is a big deal and the harder problem to solve. Clang has a head start there.
One can't incrementally improve the existing gcc frontend to be as good about error-reporting as Clang. That capability is designed in from the ground up. Incremental change there is going to take years at best and by that point it will be too late.
Right now, Clang is ahead in the user experience department but that will change quickly if the project can't figure out how to integrate support for OpenMP, UPC and the like. And not just integrate it, but DRIVE it.