If You are not using S/H circuit and only combinational logic it's possible. Then You have pure asynchronus converter, but I havn't idea what about applying any signal processing methods to them...
then how can we measure the conversion time and conversion rate for the ADC..
Due to asynchronus work of adc the conversion time should be an overal delay of comparators and logic and be differ from adc state to state.
In addition lack of clock for S/H and logic could be a serious problem
in clock based Flash ADCs.. how to measure the conversion time and conversion rate..