Thats a very cool idea. I've actually got some copper refridgerant tubing which would be well suited to this purpose. bloody hell! i just had the nicest brainwave!
the thin tubing i currently have is only about 4mm ID and 8mm OD (at a guess). But i just remembered when i was younger making one of those putter putter steam boats that are candle powered. its much thinner, probably 3mm od 2mm id.
thats far easier to work with.
it could be pretty fun making a radiator like you described with tubing like that, loads of it with paralell flows.
the problem with doing that is its a lot of drilling, and could potentially be very fiddly with so many parts all needing to be in their correct places when its solder time, and soldering would be rather tricky too with so many solder joints to make.
after your comment the wheels in my head started turning some more on the subject, and i thought of this older idea i had called "8 pipe low resistance rad" (beware, there is no 3D effect intended. its 100% 2D, cross sectional)
i think the first idea will be the easiest to make
the 6plate/pipe one would be quite easy too, and i would guess, the best performer.
I could do it by drilling 2 holes in each of those plates and the pipes they mount on, and put 2 small brass screws to hold each plate in place before it gets soldered. maybe just holes in the plates and some twisted wire would be a better choice for not having to worry about leaks, but i reckon brass screws should solder easy. (never tried)
the elliprical plate rad would probably be easier to make and a similar performer.
maybe i'll make more than one rad for fun, and test em out
thoughts?