Thanks for your helpful responses, Brad and Scott.
I was talking about the UD-BT01 rather than the MD one and you're right Scott, I had been looking at the PSR-EW400 with which that particular USB dongle is supposed to work. From their setup diagram, you use a standard USB B>A cable, into the UD-BT01 which you then plug into a USB power adapter which implies to me that it does "something" special inside (or maybe it's just that USB B sockets don't supply power whereas the others do).
Picked up the WK-7600 today.
I already have the Camera connection kit and the appropriate USB cable, but I liked the thought of not having more cables to fail/pull out accidentally at critical moments etc, esp if I want to connect to a laptop (which wouldn't be on the keyboard's music stand) rather than an iPad (which would be).
Mind you, now I really think about it, with the Y solution I'd still be pretty much wired in anyway. Even if it did work, my limited research implies I might be limited to apps that specifically support midi over bluetooth. And with wireless there's always the possibility of some rogue interference or lag.
All in all, I think your advice is sound Scott. Even if the Y dongle did work, I wouldn't get any real advantage from it.
Thanks again for your responses.
Now all I need to do is get past my weird fear of new stuff and open the box
