Good thinking, that man!

I've come across this one in Terry Pratchett's Discworld series. One character often used shouted "Good thinking, that man!" as a praise.

Is this a real English regionalism?

It's Terry Pratchett, so I think we can safely say the primary "meaning" is a humorous variation on sidekick Robin's constantly-repeated adulatory...

"Good thinking, Batman!" (as "parrotted" by myself and playmates 50 years ago)

Thus it's no more a "regionalism" than saying some equivalent like "[That's a] good idea, John!" - it's usually just a witty allusion to a 60s catchphrase. But the words themselves are just normal English, so obviously they were/are/will be used without that allusion sometimes.

But it's not common. There are just 12 instances in Google Books, of which I can only read the text in 8 cases (half of which are from Pratchett's own books). If you do hear it "out in the wild", you might get a clue from the enunciation as to whether the speaker himself is aware of the Pratchett allusion by noting whether thatman is "slurred together" into a single word.

As regards "common idiomatic usages", it's worth noting that forms such as "Well done, that man!" and "Well said, that man!" are quite natural (there are over a thousand written instances of each, in those links). But whereas it's perfectly normal to use the Batman version (or the jocular allusion thereto) when there's only the "addressee" present, it's unlikely you'd use that man in any other context, unless there were other people present at the time (the ones you're really addressing).

EDIT: Noting comments below (and the fact that a couple of people disagree with what I'm saying so strongly as to downvote it), I think it's worth considering the implications of this NGram...

It's a reasonable assumption that some of the capitalised instances will arise in contexts such as "Good thinking is thinking that is properly guided by epistemological criteria", and some of the non-capitalised instances will be things like "That was good thinking, son", for example. But it's difficult to ignore two implications:-

a) This type of "gerund" usage was far less common in C19 than C20 (so it didn't "tail off" after WW1)
b) The capitalised (possibly satirical) "plaudit" version only really took off after the Batman TV shows.