However, one simple way of implementing continuations is recompiling special forms/operators to use CPS and then defining call/cc in that terms is not that hard, and it can be done locally, without penalty to the entire implementation(just local penalty to the code defined in that manner)
This is what Marc Feeley talks about in his lecture "The 90 minute scheme to C compiler" although IIRC he just does a full rewrite of everything into CPS. He also suggests that if your code isn't going to be using a lot of continuations it is probably easier and you'd get better performance if you just to copy the stack. Do any of the CL implementations of continuations do the latter?