How to add forced water radiator?
I would call a bunch of heating guys and scrap dealers in your neighbor hood and see if you can buy another small cast iron radiator. Pipe the two in series and add a thermostatic valve (Danfoss or Honeywell) to the first radiator. It can replace the shut off valve if you have one. Then you would have automatic operation and would not have to rig up any thing else. Hot water is piped in at the top out at the bottom. The rad does not have to match the other rad ,just make sure it has connection nipples on the top and bottom just like the existing rads