Roger Long wrote:

What does ATC do when they see a 1200 target boring through what they know
is solid IMC?

How would they know, really? I think it's the other way around. I think
ATC sees a 1200 target, and they go, "look, VMC over there."

What's scarier than solid IMC is the case where visibility is perhaps around
two miles, and the IFR pilot is head down flying gauges like a good IFR
pilot, and the VFR pilot is tooling around optimistically claiming that he
can see three miles and change.

Of course most midairs happen in fine VFR weather...