Well, this has defineatley got me intrigued on whether I'm burning too much oil when running my oil burner. I contacted the Burnham people who is the manufacturer of my boiler. Basically what he said is that the hi-lo set points depend on what the boiler is being used with. This is what I got.
high mass radiant the hi-lo is 140,120 which is what I'm changing to
Low mass is 160,140
Fan coil, (toe kick heaters) is 210,190
Baseboard is 170,150
Fin tube 200,180 which is what I have
Radiators is 180,160 which is what Traderfjp mentioned. I commented that at the higher temp settings you would burn more oil. He said actually if I ran lower settings, say 140,120 for my fin tube heat I would actually end up using more oil because the house would take longer to heat at lower temps and in turn the gun would cycle on and off more frequently, and the gun only burns at one rate. Then he said, but on the other hand if the temps outside are warmer then turning down the set points would be alright. That is what the newer higher efficeint boilers do. They measure the outside temps and adjust the set points accordingly so to run at the most efficient. The setting of 200,180 would be at the dead of winter. What shortens the life span of the cast iron boilers is pretty much what Greg says. If the set points are too far apart then the metal temps will change too drastically and cause seal failure.
So, Traderfjp to answer your question the setup you want to do could work but I think you may not want to have the pump run constantly because if the water coming out of the coil drops below your boiler's low setpoint then you will be circulating cooler water into your boiler which will kick on the boiler.