Hi Summer,
A differential input of 0.8 V will not damage the device. Damage occurs when the absolute voltage on each input pin exceeds the supply voltages by 300 mV (AVSS - 0.3V or AVDD + 0.3V).
The amplifiers inside the ADS1299 will try to swing as close as they can to supply rails, but they will eventually saturate. When you read data from this saturated channel, you should see an output code/voltage indicating full-scale.
Therefore, try reducing your gain or the magnitude of the input signal. The acceptable differential input range is (-Vref / Gain) to (+Vref / Gain). For a 4.5 V reference, and Gain = 24 V/V, the maximum input is ±0.1875 V.
Best Regards,