Ranges for ADC Reference Voltages
The datasheet of the AD7176-2 is pretty careful to always refer to specific pin names, like AVSS or AVDD, rather than just generically talking about "ground".

For example, when they talk about the input voltage range (p. 4), under the line "Absolute AIN voltage limits", they give the min limit as "AVSS - 0.050" and the max as "AVDD1 + 0.05". Similarly when they give the "Absolute Reference Input Voltage Limits" (p. 5), they give the same limits. As long as you obey these limits, the chip should work as advertised. 

In particular, if your AVSS is -2.5 V, then REF- of -0.5 V is within the limits.

That said, if you can get the attention of an Analog applications engineer (like if you are doing this for a commercial project or at a university), it wouldn't hurt to show them your final design before sending it out for fab.