Where to place a radiator under a window seat?
If the existing radiator is not covered now and you want to cover it with a seat of any construction type, it will drastically reduce it's heat output. Remember cast iron radiators heat by both radiation and convection. If you cover the radiator, you will loose the radiation aspect of that cast iron radiator greatly reducing it's heat output. Radiators, were "made to be seen", and not covered. So if you cover it you may have to add more heat output into the room. This could be somewhat offset if all radiators were zoned. This may not solve the problem but it could help. Also the cover would need to have a long register mounted directly above and be the full length of the radiator to allow for the convected air to escape. I have seen many cases where covers were installed and the areas could not be heated to the desired temperature. If your heating system is hot water, and not steam, the added heat required could come from cast iron baseboard radiation, which is still made, looks decent, and is very expensive. Finned tube radiation,  not usually installed in combination with a cast iron system, could be utilized if, and only if, the whole system is zoned. On a steam system convectors and/or  finned tube baseboard units made for a steam system could be utilized in addition to the now covered radiator, to provide the necessary additional  heating for the room. Zoning would not be required if the extra radiation were sized correctly. I would be very careful in choosing the heating company you choose to do the required work. Most residential heating companies are not schooled in steam. Placement of the radiator under the cover will not greatly affect its heat output, correct installation will.