...could an eager band of volunteers/coders/hackers not "take over" Symbian development and continue to push out code for phones already out there?
If Symbian is open source then....
Nope. You can't use Symbian source code to make your own version for Nokia hardware (unless Nokia decides to allow such a thing, and release the tools and certification keys to make bootable firmware image files, which is something they in all likelihood will not do).
You can use Symbian source code for your own hardware (open hardware, or hardware you design yourself).
Aha, I suspected as much.