if your Seneca weight is 4000 pounds , and you have 800m visibility, would you take off?
What he wants to know most likely is do you have accelerate-stop and accelerate-go distances computed for that airport and ambient atmospheric conditions at that takeoff weight. Do you also have adequate runway available for that as well as a climb rate adequate for obstacle clearance in the event of a critical engine failure after Vr?  And if that’s the case, do you have a plan in place for that emergency situation?  Multi engine flying is a lot about anticipation for these kinds of contingencies.  Having a plan for takeoff and departure and anticipating engine failure on takeoff is all part of safely flying light and medium twins.  If you can’t do that, you’re in dangerous waters and the examiner wants you to think about that.

I don’t have a Seneca AFM available to me, but you should probably go back and review your takeoff and climb performance data for OEI as well as information related to your airport and departure procedures.  This should give you a much better answer to this question.

From a legal perspective, it also all depends upon under what flight regulations the flight is being conducted under.  With approx 1/2 sm visibility and flying a two engined aircraft, the departure would be illegal when conducted under Part 121, 125, 129, and 135 commercial operations, but is legal under a Part 91 flight operations.  Make sure that is included in your decision as well.

Finally, just because you CAN do something does not necessarily mean you SHOULD do something.  These conditions are pretty tight and you are painting yourself into a really uncomfortable corner with little margin for error should anything go wrong.  Are you really that competent to operate a twin, including handling an engine failure, with such low weather minimums or in complete zero-zero conditions?  Are you willing to bet your life on that knowing that once you’re airborne in that environment and on course, there’s no turning back?  Can you maintain the correct airspeed, altitude, heading, and climb rate which the AFM lists all while juggling an engine failure or some other emergency while in IMC?  Are you sure these conditions don’t exceed your personal minimums given your experience with multi-engine flying and, in particular, operation of a Piper Seneca?  If the answer is no, then you should not fly under these conditions.