I just spend a few hours messing about with Coro, googling and reading scheme documentation which got me a lot of headaches, and I'm inclined to agree. As far as I could figure out, you can't reproduce call/cc with Coro.

The fact that Coro::Cont seems to have disappeared from the Coro distribution was also kind of annoying.

I did find this interesting description of an implementation of Amb in ruby (the actual implementation is at the bottom of that page, more or less a direct port of the scheme implementation mentioned in the OP.