Connect the HDD, go to Finder and look under the SHARED location in the Sidebar, there should be your AEBS visible. Then click on it and select the HDD and double click on it. Then TM should be able to see it and use it as backup drive.
That worked for me just some hours ago.
It is HFS+ formatted, but yes, that's a good point to those who've never gotten it to work. Thanks for the heads up. I'm going to try what the poster above you said and report back.