I wrote such a library in C[1] and in practice it's been no problem. Most libraries that do IO provide hooks (for example I made SQLite fully async[2], with no changes to their code). For cases where that isn't possible (or desirable), there's also an easy way to temporarily move the entire fiber to a thread pool.[3] That's actually much faster than moving back and forth for every call (which is what AIO emulation normally entails).
Disclaimer: not production ready, for most values of "production"
Edit: stacks don't grow dynamically, of course. But that's also a problem in Go if you want to efficiently call C libraries. If you really need efficiency, you can use raw callbacks for that particular section.
[1] https://github.com/btrask/libasync [2] https://github.com/btrask/stronglink/blob/master/res/async_s... [3] https://github.com/btrask/libasync/blob/master/src/async.h#L...
Disclaimer: not production ready, for most values of "production"
Edit: stacks don't grow dynamically, of course. But that's also a problem in Go if you want to efficiently call C libraries. If you really need efficiency, you can use raw callbacks for that particular section.