I am a ASE certified master tech. I dont see any reason you couldent do this. but I would suggest more than one radiator, that's the same size as the original, which is capable of cooling the engine alone. in other words if you use two radiators one flowing into the other, circulated by the water pump on the engine, it will just lower the operating temp. This will not be detrimental to the engine. even if one radiator is on the engine(leave the original in place) with its mechanical fan. then plumb the other out to your point of use. in fact by altering flow thru the second radiator you can control heat output like a thermostat. when you,ve got enough heating, just shut of the flow circulation to the second radiator and the first (original radiator will just do its thing). a fan blowing thru the second radiator to heat air where you want it will need additional stratigy.

 Once upon a time I used to biuld race cars. it was common practice to completly remove the thermostat at the water pump 'neck" or place where the hose to the radiator connects. In your application I would leave the thermostat there. I onced asked a world class race engine builder (Ron Crawford) Hey what happens to my engine when I take out the thermostat. he seid do you have a plate in there with a hole to let some coolant thru? or did you take it completly out?- I said "no" I took it out completly. He kind of held his chin and replyied- Well gas engines make more horsepower and torque if you can run them cold, like around 130*f or so. But the engine will wear out(bearings, rings, etc) about 35% faster. Hot on the otherhand, no more than 240*f the engine just lasts and holds up longer. but in all honesty(his reply) the horse power gain is almost negligable.

 Interesting you mention using a solar water heater in the coolant circuit. this is commonly done on Gen-sets (diesel) to promote easier starting. I belive its called "heat tracing" meaning strip or wire heating elements rapped around various pipes."But" to succesfully do this an auqxilary circulation pump must be used, thats always on as long as the heating elements are on.

 It may take some work, be carfull about doing this and leaving the factory engine thermostat intact on your diesel engine. the system should work fine with it in place if its designed properly. nobody ever said "homebrewed" was easy. besides the thermostat opens when the engine really needs to dump extra heat, until then its a bad idea to take heat from the engine, by diverting cooling thru an additional radiator.
-JW

 What a great suggestion about capturing the waste heat from the exhaust stream of the engine. This is probably to most simple, and least problematic way to get waste heat from any engine, or more specifically a diesel running on Bio-Diesel.

 From what I was taught, the energy that goes into any engine (this is just a rough example)is one third of the energy that you burn actually gets you down the road, the next third of energy is used by the radiator to keep the metalurgy of the engine happy (correct coeificient of metal expansion for proper parts fit("ie" piston to cyl fit, valve train ,etc- incidently you do have to waste some heat to accomplish this) And the last third of the energy thats burned from the fuel goes strait out the exhaust pipe (or exhaust stream, meaning manifolds, pipes, etc.) excellent suggestion to tap this waste energy suppy, no modifications to the engine are nessiary, and you will get your heat up much faster. Also there is a chance that cooling the exhaust could marginally increase engine performance -due to lower presssures in the exhaust stream "ie" less back pressure, the same type of effect headers would give you for a performance car.

 By the way the idea of rapping copper pipe on the exaust is a sound one, and who ever remarked about a over pressure control on this system is thinking ahead. or a safety valve, Mc Master Carr sells a great veriety of these, all are code compliant, Id recomend one with a set psi of 15psi, possibly with a lift check feature. In this case I would recomend brazing, silver soldering, or even tin/lead soldering the wrapped copper tube to the steel pipe the exhaust is flowing thru.

 Finally when you cool the exhaust you are going to produce water from condensation on the inside of your exhaust pipe. most people do not realize that perfect combustion burn "ie" stichomitry(?) or 14.7 to 1 yeilds only water (h20) and carbondioxide (co2). like when you start your car first thing in the morning, water droplets fall from the exhaust. this is water being produced from the combustion of the engine. the only reason you see it, is because the exhaust system has not warmed up yet, and for this short time it is capable of condensing the water(h20) preasent in the exhaust stream.

 so if you attempt this cooling of the exhaust(or using it for waste heat production) be pepared to deal with alot of water comming from the exhaust stream, normally you wouldent notice it, as its ussually steam , but in this case its going to collect somewhere. be ready for it, because its not a leak in your water coil.