Now I've been thinking about this. If one was to have minor leaks in the cowel area , why couldn't some sort of rust converter be poured down there and then some sort of sealent that would seal the holes .
If water can find its way in why couldn't stuff like that ???
I agree with the others. Its not a good idea.
All other (good) reasons aside, concrete and foam trap water, as someone alredy mentioned.
So basically your cowl would become a huge sponge and corrosion would increase... .
If I had to pour something into the cowl, to seal it, it would be some epoxy coating or something similar.
This would seal everything, and it dosent trap water... .
There was a long thread on this yesterday when someone used expanding form in the cowl. The consensus was that it is a bad idea. Of course we can't stop you, but I would highly recommend that you go ahead and fix it right. Cut out the rusted area and replace it with new metal.
You know your a red neck if.... you want to use concrete to fix a leak in your car LOL
Concrete would be a bad idea IMO. Someone already mentioned epoxy. A liquid latex would also be a good candidate. However, anything that you use will probably be a temporary fix at best. Unfortunately the best cure is to split the cowl and repair.
Do it right the first time, just fix it the right way. By the time you mess around with the fillers, you could have done it the right way, it only takes a couple of hours, and then it'll be painted inside and sealed properly, no more worries.