Maybe not, but did you miss the bit in my post about FFI and GC? Have you actually tried to implement a non-trivial extension to a GC'ed language which needed to interface with C code?
EDIT: The point is not so much that it cannot be done -- the point is that it needs a human ("programmer") to do it. Which kind of defeats the point of "C-as-lingua-franca". Which, if you think about it C already kind of is, sadly. Any FFI currently in existence focuses on C already. What more do you want? ;)
No. It's a huge pain, but much less of a pain than calling between two GC'd languages. (I haven't tried calling directly between e.g. Perl and Haskell, but I'd rather gouge my eyes out with a spoon. The handles alone would bury me.) If there were one obvious GC solution, then all GC'd languages would use it, and C would not be as useful. But we've had the JVM and countless other abstractions, yet none has managed to be a higher common denominator than C, so it looks like human labor for the foreseeable future.
That's not the point. C is a great common tongue, as its function call and name lookup semantics are simple, and it doesn't insist on being in control of memory, scheduling, etc. Perl and Python are pretty similar, and both talk to C, but calling between Perl and Python is enough of an unholy mess that it's usually easier to just print text over a pipe.