I found this possible origin online, it's not quite as lively as your living doornail idea but you made me curious:

"Doornails are the large-headed studs that were used in earlier times for strength and more recently as decoration. The practice was to hammer the nail through and then bend the protruding end over to secure it. This process, similar to riveting, was called clenching. This may be the source of the 'deadness', as such a nail would be unusable afterwards."

http://www.phrases.org.uk/meanings/as-de...

Apparently the phrase dates back to at least the 1300's and was used by Shakespeare.

I don't know about door nails, but I do know a few door knobs....you know...everyone gets a turn.

If they were ever alive, it might get confusing. Like 'dumber than a box of rocks' might cause one to wonder if a basket of rocks was smarter, and then a bag of rocks, and then a handful, a riverbed, a gravel pit of rocks....

I heard that the expression came from the time when
nails were routinely re-used. . but nails which had
been used to build doors were bent over after being
nailed through the wood, which made them unavailable
for re-use. -- j

A Leftist Approach: Since everything in the Universe is made of the same stuff, who is to say what is dead or alive? Nails should have the same rights as everyone else.

I am more interested in "knockers" but them I have always been accused (rightfully) of having a prurient sense of humor.

Door nails were the first victims of Global Warming. How many more nails have to suffer???