Can you get the 3.3V supply out of a very good LDO linear regulator, so the supply doesn't vary at all, and so everything has a very solid 3.3V supply? Then, you can use very tightly-toleranced (even 1% may be alright) resistors to make some resistor dividers from the LDO output to get your reference voltages. I've done this, and it works quite well.
If you are *intentionally* varying the 3.3V supply, though, then this isn't a solution to linearly track the supply voltage...not sure why you would even begin to design a DAC with the intention of not having a steady supply voltage, though.