The Vinculum chips are far easier to implement than a "standard" USB host controller. They actually
might be a solution for the QL. But I go with Peter in saying "Solution? - To what problem?". Keyboard/Mouse is not really a problem, storage neither, and printing should be solved with something like RWAP's retro-printer. That leaves networking, but unless someone comes up with a really powerful CPU upgrade, that is most probably going to end in disappointment.
If you want to come up with something generic that encourages the tinkerers to come up with peripherals, USB is too complicated IMHO, to motivate someone to put together a tinkerer's peripheral. In case that is your goal, come up with a generic SPI interface supporting an address range of like 4 devices - The embedded world has some interface standards that fit much more to a rather slow device like a QL than the modern desktop computer world which is already lightyears away.
To Peter's comment: QDOS is really designed to handle peripherals in a
polling way in the polling interrupt. That doesn't mean it wouldn't support direct HW interrupts, but can give you a bit of a headache when doing it. And I do agree with you that non-crackling sound is basically impossible without interrupts. There it's a MUST. And the Qimi design really is a design fault
Tobias