Ported the Si5351 clock generator library from Adafruit today, and made a nice xC interface for it that links up with XMOS' lib_i2c. I can program arbitrary frequencies on all 3 outputs over i2c from the xCore mpu, no problem. I will publish this on git very soon, after a clean it up a bit more.

Right now I'm using the little 3-output board made by Adafruit, but I will be assembling one of my own 8-output boards soon to test as well. It should work fine with the existing code, and a few tweaks to enable outputs 4-8...