As for having experience, I am a senior SW engineer at a large telecoms company (I write embedded RT stuff), I have an electronics degree,

Ah, good. I mention it because sometimes people will come here and spout off pie-in-the-sky ideas without any real notion of how to get the job done. Sort of like walking into a bank and shouting "I sure wish someone would give me some money."

But those credentials seem to indicate that you know what you're talking about. That's a good start. Now can you detail how an interface box might actually allow a USB slave port to magically behave like a USB master port?

Regarding the "killer app": Look at the list of things in the FAQ that you "can't do" because of USB-slave and decide what would be the most useful.

For the Mark1 player, I think that would be allowing a USB-to-ethernet or USB-to-802.11 converter to be attached to it. For the Mark2 player, I think the killer app would be attaching a USB CD-ROM drive so the player could do Jupiter-style realtime ripping.