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

Nothing. They don't *know* it's IMC where the VFR target is on the screen.

The only time I've ever heard an ATC comment about something like this was
west of Houston one day. A pilot reported climbing through a solid cloud
deck at the same time another pilot was reporting "jumpers away" from his
meat bomb hauler nearby. The controller said something like "I don't know
who it is for sure, but somebody's fibbing."
--
Dan
C172RG at BFM