Hi, Steve.
A clock is very important to a converter. It is used to tell the part when to consider the data to be valid or not. Without it, the part should still have a way to tell whether the data that is sent to the register are valid or not. One way is to use manchester coding.
I could imagine DAC's using manchester coding but unfortunately ADI do not have clockless DACs. You are correct on the AD7801. You may also take a look on DAC8562.
Regards,
Mark