This is what I did (as usual, not many details, what worked for me may not work for thee, more hints in the usual place, etc):
I installed rooted WSA
from here, since it does not seem to have many ads. Then I installed K4A 8.123.0.100 from google play store, and logged in and downloaded a few books.
Then I installed frida server (have to be careful about architecture, it has to match host, not emulated ARM), and ran it from root shell (KernelSu).
Then i connected instrumentation script and opened book. It was slow, and occasionally crashed, but usually only after it printed out the key for that book (several times, same key). I then put key string into file that was spat out by previous tool (though empty text file would also work, of course), adb pulled book folder, fiddled a bit with deDRM and successfully imported the book.
@lkmiller, yes, that should work on the books with "heavy" encryption, until it is blocked by Amazon again... so for another week or so, if we are lucky? I checked with one book that I have.