Using the whole front end or undertray as a coolant "radiator"?
 

What about using the whole front end and undertray or at least the metallic hood and fenders as a radiator by running coolant tubes beneath them?
This way the front grill might be closed completely to reduce drag. Maybe only requiring a small opening for the AC radiator.

Of course

-This will increase weight and require more coolant.
-The front end might get too hot to touch specially for children and dumb adults.
-Body paint might hinder heat exchange and the paint itself might be damaged from heat.
-A dent might rupture the cooling tubes, spilling coolant thus needing to tow the car away.
-To open the hood some wear-prone flexible coolant hoses would be needed. BTW some insulated handles can be fitted allowing to open the hood when hot.
-Hoods already get too hot from the sun. Maybe this restricts the idea to a flat undertray.

Has this been tried or proposed before?
Maybe for airplanes or some other vehicles?

Yes, the idea has been discussed and for the safety, complexity & weight issues has been dismissed, some aeroplane makers have tried it in the past, but it's hard to beat the weight efficiency of modern radiators.
If you could channel the heat efficiently and eject it at the rear that would provide some drag reduction benefit, but still presents many problems.

If your grill block is designed properly, you will have recovered 80 - 90% of the drag problems there, trying to scratch out that last bit will require a lot of effort for not much gain.

Have you sealed all panel gaps into engine bay so the only air entering is through radiator?

This is quite the demanding and time consuming task! I've started though... :)

another issue i see is that the air flow accross the hood is laminar, which isn't too good for heat transfer. The turbulent (i assume) air through the radiator would do a much better job.

What you need is an undertray made from dimpled heat exchangers that are normally fitted to vessels for liquid cooling/heating. Also used on the underside of boats.

I used duct tape and expansion joint foam, or yoga/camp mat type of foam.
In some areas where gaps were irregular, like behind headlights, I used softer foam over the entire area, then stuffed in cushioning behind it and also used those water bladders from boxed water, just pushed them into the area then inflated to fill the gaps.

tried
 

The 2011 IVECO 'Glider' semi tractor is doing something like this.There's no 'radiator.'

If I remember correctly Brabham used this briefly in F1 cars in the '60's or 70's. What I remember was that they couldn't get this to work well because the surface area exposed to air is sooooo much higher with a flow through radiator (orders of magnitude!). I don't think the issue of laminar vs turbulent flow was the deciding factor.

Edit: It was the Brabham BT46 - 1978. Never used in a race.

Porsche's Type 718 (AKA the RSK Spyder) used something similar for cooling its oil.

From a blurb about one of Karl Ludwigsen's books--

Might be good enough for the relatively low demands that are placed on the cooling system by driving for economy. Not good enough for the worst-case, though.

-soD