Ok, dug out the details. Here's the previous attempt at an app like this:

http://www.mulliner.org/bluetooth/xkbdbthid.php

And I had it backwards: Apple requires that the keyboard support MORE than just "boot mode". if you search for "Mac" on that page, you'll find this note:

"Supports ONLY Boot mode (this means no support for MacOSX, yet!)"

Since it's also based on the BlueZ library, I don't know if it's a library limitation, or something else. But, there's all the info I have :-}