AFAICT, it is disabled by default, and it is not documented in the ASDF manual.

So is this code even live, except for in the test scripts?

It was pretty painful to get this to work because it relies on unexported (and hence unstable) SBCL internals.

My guess is that this was originally done as a way to dispense with WITH-COMPILATION-UNIT and hence potentially parallelize the compilations.

If no one is even using it, I am tempted to move it to be a contrib and drop it from the tests and maintenance as part of core ASDF. Maybe it would be better as a part of a POIU extension to ASDF, and relieve me of dealing with these SBCL compiler internals?

Cheers,
r