Status update: I did decide to look into how exactly ADE stores its activation, and the next version of the plugin will also support importing an existing activation from Adobe Digital Editions into the plugin without using up an activation. I have tested this with ADE on Windows and ADE on Mac (they both needed completely seperate implementations ...) - it will not work (yet) if you're on Linux and you are running ADE in Wine.
Thanks jhowell for poking me and suggesting that again
With that new update (not released yet), there will be a new button "Import activation from ADE" in the plugin settings when it's not authorized. If you're on Mac, you will need to enter your keychain password when asked. If you're on Windows, it'll just work by clicking the button.
Now, the next interesting thing would be taking a Calibre activation and
writing that back into ADE... That would mean that you could basically use Calibre to copy one single ADE activation around to multiple different ADE installations on multiple different machines ...