I added the Bookmacter chrome extension to Edge Canary, but there still seems to be no way to add it to my list of clients.

Hello, hmgnsd.

Thank you for contacting us.

I was not aware that Microsoft is bringing Edge to macOS.  I just downloaded Edge and verified that, as you said, it's another Chromium, and although adding support for it is a little more complicated than flipping a switch, it should be less than a days' work, so this is probably something we would do soon.

I don't know whether or not we would put the necessary extensions (BookMacster Sync and BookMacster Button) into the Microsoft store, but I see that Edge allows you to install extensions from the Chrome Web Store.  So if we get lazy, and/or if the Microsoft Store turns out to be too much of a hassle, we may require Edge users to get our extension(s) from the Chrome Web Store.