Powering TL431 from MCU output possible?
Yes, I think this will work, at least up to 85°C . Output drop is specified as as &lt; 800mV at -10mA and 3V supply, so with 2mA and 3.3V supply the drop should be &lt; 180mV. 

If the supply is 5% low (3.135V), the resistor 5% high (409.5\$\Omega\$), and the reference at the maximum 2.550 the current will be 0.99mA. So maybe use a 1% resistor, but 5% will be fine too.