I advice you to use TI or SMSC chips for this purpose.
I think that no change to the chip is required. The only thing to check is basic UTMI compliance and if the wishbone interface is good for you.
So what you need is a solution with not only USB 2.0 phy but also with SIE and endpoint logic
I can suggest you to take a look at the Cypress EZ-USB familly. http://www.cypress.com/?id=190&source=header