I like the universal IO board concept you've come up with here, well done!
Your choice of using the MAX525 is understandable as its already supported by the MBHP. I assume that you've come across mxmxmx's Ornaments+Crime module? He's using the TI DAC8565 which has 16 bit precision. I do understand the trade off here of 4 bits for THT but this is worth it in this case.