I have a window seat over a long, horizontal old cast iron radiator in my circa 1914 home. It is well vented in front using decorative screened metal and sides, top are wood... Hope that helps. There is always a way, with some creative thinking!

You can have your contractor redirect the vent with a simple attachment that goes from the floor to the toe kick at the base of your window seat. It is done all the time with built in cabinets placed over floor vents. It will go from a horizontal vent to a vertical vent.

Radiators emit the majority of their heat up rather than out or down. We have an old 1920's home with numerous radiators and most were covered with metal or wood covers that had front screens but not top screens/openings. We got rid of these covers and are amazed by how much more heat they now throw off. One solution is to build a wood ledge that hangs over the radiator and is 3 - 4 inches above it with a few post supports if needed but does not "cover" the radiator like a bucket. Drill a bunch of 1 in or larger holes in the ledge to let the heat up and then put a thin removable cushion on top of the ledge.
OR
Dont build a ledge but just put a custom cut piece of Pyroblanket on top of the radiator with a thick cushion on top of that pyroblanket. In both cases you have to paint your radiator to give it a finished look and you'll be surprised how much character it gives a room.