I wish to recover the license key for use on a replacement machine as my original machine was dropped and now will not even boot.  How can I do this given that the standard way is to run Macrium and click "Remove License"?

Contact Macrium Support and ask them to reset the activation on the license key. You can still activate the new system in the meantime though. I ended up in this situation with a new server when I had to rebuild it a couple of times during a migration project that had some hiccups and I forgot to remove the Reflect license before I wiped the server each time. I heard from Macrium about it, but I explained the situation and they sorted everything out.