I am still waiting for all my favourite applications to be extendable in GNU Guile. On a more general note, I am still waiting for the world to fall for lisp in general.
As someone who's spent the last week implementing two different Schemes atop two different languages (Lua & Go), I think I can explain why GNU Guile didn't take off: Scheme. The problem with Scheme is that it's an utterly lovely little language with some wonderfully neat concepts which is great for teaching students about continuations … and it happens to have some broken features, like DYNAMIC-WIND instead of UNWIND…
And dynamic-wind is the price you have to pay for call/cc. UNWIND-PROTECT is not the same thing. Multi-shot continuations is he reason. One way would be having explicit one-shot continuations and escape continuations. Those are faster and will make UNWIND-PROTECT work.
I am more in favour of delimited continuations though.
Delimited continuations makes that a bit simpler (since you can easily implement something like dynamic wind using them).