I don't think DMA is being utilized in DUE TWI functions. As noted in the ADC PDC example in this thread, somewhere there needs to be code that is setting the PDC address and byte-count registers for the DMA transfer. I think your interrupted print, only shows that a TWI interrupt occurred after a byte transfer. DMA can send many bytes, requiring only one interrupt when all of the bytes have been transferred.
Though the Maple and DUE share the ARM CPU architecture, the peripheral architecture (I2C, DMA, etc) are radically different. So my maple I2C DMA code would not be very enlightening. Eventually, I may make a github page for some of my maple hacks...