squeezebox duet I2s mods?

Clearly, your best solution will be to purchase the Bridge from PS Audio once it's released. The cost of an I2S mod (if it can be done - and I think Cullen Circuits was looking into doing I2S mods) would likely be the better part of what the new Bridge will cost (approx. $500) - and the "Pacecar" will cost you a lot more. The Bridge will allow up to 24/192, while the Duet is limited to only 24/48. The Bridge is a superior option for building a high-end music server in conjunction with the PS Perfect Wave DAC. I am currently running my Duet through the PWD until the Bridge is available.

You could run the Duet into an Empirical Audio Pacecar via S/PDIF and get I2S output from the Pacecar.