If you are going to use "low pressure" messages on the ATIS then it should start at 999mb and below, on the basis of the fact that the pressure has to be read back as 999 millibars until it is 1001 or above.
Personally I dont see any advantage to putting a "low pressure" advisory on the ATIS, the QNH has to be read and acknowledged at the point the aircraft is descending to an altitude and further as applicable.
Equally, most of us are operating on SSR equipped radar, and at my unit, and several others I know of, it is written in the MATS 2 that the SSR readout shall be monitered, by verbal or visual confirmation via the radar, on descent and climb out to ensure that the correct pressure setting is applied.