RO requires constant high pressure, if you can, have the tap always turned on to the RODI unit then have a solenoid on the outlet that controls filling your topup tank (via float switch or similar) then, have a dosing pump on timer (or another float switch) to top up your system - that would be the best scenario IMO.
you could always have potential issues with the timer failing, or the float switch getting stuck or similar so it should never be fully automated and checked regularly. options to build redundancy into top up are pretty much endless, you could even go as far as having an intermediary topup tank (of fixed size) between your topup drum and sump with another dosing pump on a timer to ensure you never have a drop in salinity because of a dosing pump or timer stuck on!!! depends how anal you want to be
for me, importance is with heat at the moment - i want to add a thermal shutdown to my system so the halides switch off if the tank hits 30 degrees. this is because the room that my in-wall tank sits in will be enclosed soon (alot smaller) i want to add extraction fans on thermal controlled system as well so they will come on if the tank hits 27 degrees. chances are it should never turn on as the chiller should maintain the temperature around 25. thats why i put the sump room and halides on the same RCD circuit, if something in the sump room trips the power, then the lights (main source of heat around the display tank) will go off also. the closed loop is on a separate RCD to maintain circulation. if the power goes off to the whole house, both the room for the display tank and the sump room are insulated so the tank temperature should maintain constant for sometime.