Well I think I found the source of my problems, though it does not explain why SD.Begin() hangs.
I had read that the USART on the 1284P could be used for another SPI but at no point was it mentioned that it uses different registers and works a bit different from the regular SPI, so I can't simply change the SPI pins to those of the USART unless perhaps I am using a soft-SPI where the pins are toggled manually instead of hardware SPI.
Anyway that means that it's likely I will have to go through all these libs and figure out how to change all the registers to make them work properly with the USART. 
If only I'd known this would be an issue I'd have stuck my DAC on the USART and the SD card on the SPI.