It would be awesome an art historian would look at a bunch of paintings for those defects, or if someone could work out how many portraits are 1:1 scale, like Tim's second "Claire with a Pearl Earring" experiment.
Now imagine that we found a half-burned-down painter's workshop in the countryside, with a bunch of artifacts. Or a woodcut or drawing of a painter's workshop with these devices in use. Those would be very convincing to a critic. But we don't have those.
Hopefully we will find a workshop somewhere with a bunch of photographic equipment and crack this puzzle.
And their existence is part of critic's arguments against the Hockney thesis: many tools have been used in various ways at various times, and there's been little or no shame in acknowledging and documenting it.
To accept Hockney-Falco, you'd have to accept the opposite: that such devices were widespread amongst artists but kept secret or never mentioned. I think Occam's razor suggests that if such devices were commonly used, it wouldn't be so hard to find evidence of it.
It looks like the article preceded publication of Hockney's book and originated at a time when he was still rehearsing and refining his argument.
His main antagonists in the article are the art historians who come off a bit like stuffy reactionaries. I was firmly in Hockney's camp by the time I had finished the article. It was one of those ideas that changed the way I look at the world slightly.
Never read the book. Tim's Vermeer was the coup de grace for me and I was gratified to see Hockney make a cameo in it.
There's virtually no discussion of the design of Vermeer's paintings, which was elaborate to say the very least. I was so hopeful that time would apply his technique to some other subject, but it stays almost myopically focused on the rendering process.
The reason -and it's a technical, not a philosophical reason- is that theories
like this are really trying to determine a process from examples of its
output, where the output in this case is some behaviour of a living organism
(a human artist creating a painting). Unfortunately, for a sufficiently
complex behaviour there may be any number of processes capable of reproducing
it to the same degree of fidelity- "any number" as in an infinite many .
The best you can hope to do then, is to approximate the target process to some
arbitrary measure of error. You can easily measure the error by comparing
examples of the target process with examples of an approximator. However, you
cannot prove that an approximator _is_ the target process- not even given 0
error in the reproduction, because finding one approximator is more of a hint
that there are more approximators possible than any sort of proof that the
approximator you found is the only process capable of reproducing those
examples with that error.
So the problem with the Hockney-Falco idea is that there is no way to tell
whether a work of art was created using an optical device, rather than
without. Only that it could have. And that's really the strongest claim that
one can legitimately make: "the Old Masters could have used optical devices to
create their masterpieces". From there to "... and they actually did" is a big
leap- of faith, not reason.
 There's maths on that- learnability results, particularly related to
automata induction and so on. I'll dig out the refs if anyone is curious. But oh god, please don't :)
It's unlikely that so many masters would use this technique without leaving some evidence of it behind. So, in the absence of that kind of evidence, I think it'd be okay to dismiss it as a primary force of innovation during the renaissance, especially if other techniques exist that were more readily discussed.
At least it allowed a lot of non trivial math (computer vision).