>>79
CL-CONT - portable library, code walker, and implements call/cc via CPS
arnesi - portable library, has a code walker, and implements call/cc via CPS
I also see 2-3 other, less serious, more toyish implementations of call/cc floating around.
One problem with stack copying is that it's not enough: symbols can have their own values, which are stored on the heap. It is possible however to use heap copying: there is a SBCL fork which implements a contrib called SB-HEAPDUMP which does just that and can be used to implement such native persistent closures. I suspect it's doable in some other implementations as well, but I haven't looked into it.
Offtopic: "The 90 minute scheme to C compiler" seems like an interesting experiment I'd like to try. It reminds me of that book "Lisp in Small Pieces" by Queinnec.