Who makes hydronic radiatiors to fit in a kickspace? I would like the airflow to be passive(convection) not powered by an electric fan.

Some sort of long continuous finned tube?

No one I know makes a kick space heater without a fan. The units are small and without the fan would not put out very many BTU's of heat so no one makes them. If it is in a bath many people use a hydronic towel heater. They have no fan but do put out some heat. It is also nice to have warm towels after the bath or shower. Years ago my wife told me to put one in and it does work great.