I added some simple Kyua tests to exercise the code.
I also have an experimental fork of PostgreSQL that uses AIO and consumes completions with this mechanism, at https://github.com/macdice/postgres/tree/aio-kqueue, which has motivated my work on this; this should be compiled with --with-posix-aio and run with io_method=posix_aio, and then big I/O workloads can be run through it to exercise it.